Most shoujo manga series require a love triangle at some point. And it's not only shoujo: character-driven shounen (like KOR, mentioned below) also rely on this heavily. Heck, even non-character-driven shounen have them sometimes.
The anime TV series Kimagure Orange Road is built partly around the long-term problems and implications of the triangle between Hikaru (A), Kyousuke (B), and Madoka (C). May have been the Trope Codifier for anime-based love triangles.
In Martian Successor Nadesico where our main hero, Akito, is practically pummeled by affection by nearly every female character in the show, the main contenders being Yurika, Megumi, and Ryoko. Yurika has the distinct advantage of having known him at some point during their childhood, but her attempts to impress him are in vain for the most part. He actually appears to be going steady with Megumi in the 4th episode, which concludes with them kissing and walking along holding hands in mutual affection. However, their mutual attraction seems to begin and end there, as the Clingy Jealous Girl routine disqualified her later when she tried to move their relationship into adult territory. Ryoko is a more subtle contender in that she has strong feelings for him, but doesn't actively seek him out due to her pride. In any case, she seems to have garnered more attention from him than either of the other females (albeit not enough for him to see her as much more than a friend). Also, he does develop feelings for Yurika at some point, though her Clingy Jealous Girl antics are what kept the romantic flame stunted throughout the series. He winds up with her at the end as a result of finally caving in to his [somewhat coerced] feelings for her.
Also, given the parodic nature of the show, it was perhaps a subtle jab at harem anime, rather blatantly implying that the guy just might fall for one of the girls in the harem if they all weren't so aggressive about garnering his affections.
On Keroro Gunsou, Tamama (A) vies constantly with Angol Moa (B) for the attention of Keroro (C), who seems somewhat oblivious of Tamama's semi-platonic jealousy and Moa's crush. Keroro Gunsou even has a scenario with both types of Love Triangle rolled into one: Giroro (A) and Koyuki (B) both have a crush on Natsumi (C), who has a crush on Saburo (D). Unlike most of the above examples, the triangles aren't central to the plot of the series.
Another one involves Giroro's kitty Miss Furbottom (A), Giroro (B), and Natsumi (C). The narrator is quick to lampshade it.
Mermaid Melody Pichi Pichi Pitch had Lucia, who was Kaito's girlfriend, but then Michel took Kaito's memory, leaving him open to Michal, who was Michel's symbiote. Meanwhile, Lucia's trying to find Kaito, and when she does, get him back.
Berserk has used this trope repeatedly throughout its run. The major one was the triangle of Guts, Casca, and Griffith. Guts's and Casca's romance with each other is undercut by their (separate) loyalty and attraction to their leader, Griffith. Said attraction is emotional, with implied physical undertones in both cases. Griffith himself seems incapable of entirely returning either's feelings, but does care deeply for both and shows a certain amount of emotional and physical attraction to Guts. No, it does not end well. Later, the initial triangle is echoed in the relationship between Serpico, Farnese, and Casca: Serpico seems to be in love with Farnese, who needs him very much but seems to be paying more attention to Casca. There are hints of a triangle between Isidoro, Schierke, and Guts as well. However, if you add Judeau to the first triangle and Roderick to the second one, it begins to seem more like a Love Dodecahedron...
A love triangle would be between Guts-Casca-Farnese. Currently, Guts is still completely in love with Casca, who unfortunately can't love him back due to a series ofunfortunate events, but he is devoted to her and only her and it is, in fact, the whole reason why Guts is on his current mission. Farnese, on the other hand, is in love with Guts, but he doesn't see her as anything more than Casca's caretaker. While she cares for Casca a lot (which brings on some more implications) and she knows that it really isn't Casca's fault for acting like such, Farnese still can't fight with her inner feelings that Casca is ungrateful toward Guts which causes some resentment on her part. Wonder how that's going to turn out...
A minor but still important triangle exists in Slayers between heroine Lina, her swordsman partner Gourry, and Sylphiel, a priestess and old friend of Gourry's. In this case, it's mostly one-sided, because not only is Gourry dense towards Sylphiel's feelings, he openly gains affection for Lina as time passes.
And then the Love Triangle is turned on its head when Yuuko vanishes, and she's the one Watanuki is endlessly pining for instead while Doumeki remains as loyal to him as ever. Meanwhile, the situation becomes such that Himawari can only see Watanuki in person once a year and winds up married to someone else. And Doumeki marries his and Watanuki's common friend Kohane Tsuyuri, who's also hinted to be in love with Watanuki, presumably to comfort each other and have descendants that will keep Watanuki company afte their deaths.
A Love Triangle is practically a requirement for a Macross show. Each and every installment has featured one.
Macross7 is notable in that Mylene has a dedicated, loyal suitor in Gamlin, yet is unable to decide between his obvious affection and the brash emotional personality of Basara, who by all indications isn't even aware of her potential feelings for him and generally irritates her more often than not. And besides that, Basara's not only attracted the obvious romantic attention of a biker girl secondary character, but is locked in a pseudo-love triangle over the Enigmatic Minion Sivil, with his competition being The Dragon of the series' first half, Gigil.
One of the more audacious moves is the one seen above in, Macross Frontier, where Alto gets the attention of two Idol Singers Sheryl and Ranka (above). What's audacious about it is that, despite dispensing GALLONS upon GALLONS of Ship Tease so that it could've gone either way, the Love Triangle is never resolved in the show. Every subsequent iteration nonetheless features the Love Triangle quite strongly (and some actually end it).
The Wings of Goodbye: And finally, after years of this, the second movie resolves it, though every iteration is technically Alternative Continuity.
The principal love triangle of Full Metal Panic! encapsulates the primary female leads of the series, Kaname and Tessa, and their individual versions of affection for male lead Sousuke (who is unaware or just beginning to understand that he's in love with Kaname, and mainly sees Tessa as his superior officer and friend, much to her frustration).
Plus the fact that Sousuke and karate club president Tsubaki can hardly stand each other is undoubtedly fueled by the fact that they're both smitten with Kaname.
Around the same time it's made fairly clear that Tessa is something of a Hopeless Suitor, her brother starts showing very clear interest in Chidori. This element dominates its fair share of the story's later drama.
Depending on how you look at it, there is a weird, warped sort of love triangle between Sousuke, Gauron, and the twins Yu Fang and Yu Lan. With Gauron being completely and obsessively in love with Sousuke, the twins loving Gauron, and Sousuke hating Gauron. The most important triangle is the one between Sousuke, Kaname and Gauron. Except in this case Sousuke wouldn't have to chose between two people, but rather between two different life styles and/or two versions of himself: the weaker, gun-toting, socially clueless version that he is with Kaname, or the cold-blooded, unstoppable killer that Gauron became fascinated with once upon a time. All this became rather obvious at the end of the first season, and The Second Raid has only managed to take this further by having Gauron trying to Murder the Hypotenuse.
Nobuta Wo Produce: a classic high school Love Triangle, notable because the three started out (and eventually ended up) as close friends. Akira was head-over-heels in love with Nobuta, but Nobuta had a shy crush on Shuji. Shuji and Akira also had a ridiculous amount of Ho Yay played up between them.
Kaze no Stigma: Ren with his two best friends (a guy and a girl), and Kazuma with Ayano and Katherine (both type 3).
Much of ef - a tale of memories revolves around the love triangle between Kei, Hiro and Miyako. Miyako wins. Kei may or may not get together with Kyousuke.
Super Robot Wars Original Generation series, has Mai Kobayashi, and Latooni Subota both going after Ryuusei Date. Who does Ryuu love? Latooni's giant robot. That, or Lune's giant robot. Or Lamia's. Or maybe Aya's once it got equipped with a skirt...
Kusuha might have been in there at some point, but she was destined for Bullet by game of origin.
Prétear had a Love Triangle in the backstory(Sasame-Takako-Hayate), although it isn't revealed until much later. Then there is Mawata's crush on Sasame — and Sasame, depending on the adaptation, either likes Himeno (who likes Hayate), or still loves Takako, enough to betray his friends and join her.
Princess Tutu has two. First there's Ahiru/Mytho/Rue, Ahiru has a crush on Mytho but he's in a relationship with Rue. However, with his heart coming back together, he falls for Princess Tutu, who is Ahiru. This in turn, causes Rue to turn evil.Whoops. Also implied but never stated is Fakir/Ahiru/Mytho.
Fruits Basket has a not-exactly-love-triangle... Kyo and Yuki never precisely compete for Tohru, but they both care about her and their rivalry sometimes extends into this. (Creating multiple Crowning Moments.) It shifts to Kyo/Tohru/Momiji about halfway through the manga, which plays the trope completely straight.
Then there was the twisted Love Triangle in the manga, which involved Shigure, Akito and Kureno. It was solved in the end: Shigure and Akito get together, while Kureno goes with Uotani.
Averted in Bakuman。 as Takagi shuts down the love triangle between him, Miyoshi and Iwase as soon as he's aware of it, remarking how stupid the whole situation actually is. The possibility of it becoming a distracting Plot Tumor is killed from the get-go, unlike in a lot of other Shonen manga where the Love Triangles are allowed to linger for years without contributing to the plot or showing any signs of being resolved. The author must really dislike Ship-to-Ship Combat...
In the second Kujibiki Unbalance there is a love triangle between Chihiro, Tokino, and Ritsuko. This does not apply to the first version since in that version, Ritsuko is the only one that has any romantic involvement with Chihiro. However, due to it being only 3 episode out the supposed 26, it may be possible that there was some kind of love triangle and Ritsuko simply won in the end.
Mobile Suit Gundam has Amuro, Lalah and Char; Amuro, Fraw and Sayla; Amuro, Hayato and Fraw; Amuro, Sayla and Lalah... Damn it Amuro! There's also Bright, Mirai and Cameron (and Sleggar). In the end, Amuro accidentally killed Lalah, who was protecting Char. Fraw hooks up with Hayato. Sayla ends up with no one. And finally, Bright ends up marrying Mirai and they have children.
Except for promotional images, Gundam SEED's main triangle are Kira, Flay and Sai. Kira and Sai patch things up and moved on with their lives, plus Kira ends up with Lacus. Flay on the other hand dies in the end.
Gundam SEED Destiny has many courtesy of Athrun's harem. Damn you three Athrun! In the end, Luna goes with Shinn, Meer dies, Cagalli breaks her engagement to Athrun but it's left open-ended so it's rather likely that they'll get back together when their work is over, and in the meantime Meyrin and Athrun are very close friends so it seems that, if Cagalli decides to stay Marriedto The Job, she may have her own chance.
Gundam Unicorn has two: Micott, Banagher, and Audrey and Banagher, Audrey, and Riddhe in the light novels. Only the latter love triangle appears in the OVAs.
Gundam Age has Flit with Emily and Yurin. Damn you, Flit! Emily wins and becomes Asemu's mother in the end, since Yurin went the Stuffed In The Fridge way. And she's painfully aware of it. Asemu gets his own love triangle, involving Romary and Zeheart. Romary seems to choose Zeheart at first, but by the end of the second generation, marries Asemu and they have Kio. Zeheart goes the Anti-Villain way and ultimately dies.
Naruto: Mainly the Naruto/Sakura/Sasuke love triangle. Naruto loves Sakura, who loved (and might still love) Sasuke and might love Naruto. It's still unresolved so far and it's really complicated stuff.
Technically, Naruto/Hinata/Sakura to a lesser extent. Hinata loves Naruto, who loves Sakura, and who Sakura might love too.
Sakura/Ino/Sasuke mostly in Part 1. Sakura loved Sasuke and Ino liked Sasuke too, leading into a rivalry.
Kakashi/Rin/Obito. Obito loved Rin who loved Kakashi instead who didn't have feelings for her, leading to messy but brief love triangle.
Lee could be thrown in too with the Naruto/Sakura/Sasuke love triangle. Lee loves Sakura, who Naruto also loves, and Sakura loves Sasuke but might love Naruto too.
Shonen Jump Question:
Naruto and Sasuke kiss (accidentally) in the first chapter where they appear together. Does this add a special dimension to their rivalry?
Masashi Kishimoto Answer:
I didn't have any particular reason for it, other than to surprise readers. I mainly did it because I don't think there has ever been a manga where two rivals kissed. Also, by having this encounter, it was easier to set up the love triangle: Sakura, who had intended to be the first one to kiss Sasuke, has it stolen by her rival Naruto, Sasuke and Naruto are rivals, so there's and added tension there. And so on and so forth.
Neon Genesis Evangelion: Asuka/Shinji/Rei. Shinji deeply cares for both girls but he does not believe anyobody could love him. Asuka likes Shinji but she has a hard time admiting it to herself, partially because when she flirts with him, he is clueless, making her believing he is not interested in her. And Rei does not know/realizes her own feelings and the fact of being Shinji's mother's clone is, no doubt, a complicating factor.
More disturbingly, there's the End of Evangelion's Love Triangle between Gendou/Ritsuko/The Magi Supercomputers. Also, if you want to get into hopeless Love Triangles, there's Ritsuko/Naoko/Gendou/ Yui / Fuyutsuki ( Rei is Yui's partial clone. And therefore Shinji's fraternal twin half-sister. Above "normal" A/S/R Love Triangle just got very very squicky.) and Misato/Kaji/Asuka/Ritsuko. Though that Love Triangle may just be an attempt by Asuka to attract Shinji's attention. Yeah... Have we mentioned Eva is weird?
Ichinensei ni Nacchattara: Three ~7 year olds: Miho —> Touta —> Iori. Iori, secretly a genderbent highschooler, wants no part of it.
Kenichi: The Mightiest Disciple has a minor one between Kenichi, Miu, and Renka, although it's fairly obvious that Renka doesn't have much of a shot. Later on, there's also a possibility of a Kenichi, Miu, and Shigure triangle.
Gantz Has one between Kei Kurono, Masaru Kato and Kei Kishimoto then one between Kei Kurono, Tae Kojima and Reika Shimohira but later the second Kei Kurono makes it into a love rectangle.
Honey & Clover has two main love triangles (and a third one in a flashback): Takemoto and Morita both fall in love at first sight with Hagu, who is oblivious to their feelings; Yamada is in love with Mayama, who in turn is in love with an older woman named Rika; in the flashback, Shuu was in love with Rika, who was married to Harada (who was Shuu's best friend and died in an accident).
In the end, Hagu does not return Takemoto's feelings and instead falls for Morita, but she chooses to stay with Shuu (her uncle) because she believes he'll be able to take care of her better; this love triangle is left unresolved and open-ended because although Hagu chooses to stay with Shuu, she is still very much in love with Morita and vice versa. Mayama wins Rika over, and Yamada is implied to find a new love in Nomiya, a guy who she works with and ends up helping Yamada come to terms with her unrequited feelings for Mayama. While Rika felt mutually about Shuu in the past, they decided it was best for them to stay apart because Harada's death made both of them emotionally unstable (after it is implied that Shuu almost pushes Rika off of a rooftop because he believed that the Rika, who became depressed over Harada's death, would probably be much happier with Harada).
Samurai Champloo - It's been said that Mugen, Jin, and Fuu all have crushes on each other.
Scryed had one between Ryuho, Mimori, and Scheris (which was part of a Love Dodecahedron if you take Kazuma and Straight Cougar into account!) In an ironic turn of events it ends with Scheris dead, leaving Ryuho and Mimori to possibly hook up, but Ryuho refusing to accept Mimori's advances and using Scheris dying because of her love for him as the reasoning for this. Scheris must have been laughing in her grave.
A brutally twisted version of this occurs in Mirai Nikki considering how two ends of the triangle, Akise and Yuno, are both Yandere (Yuno even more so) for Yukiteru. However, in chapter 51 Yuno stabs Akise in retaliation for him kissing Yukiteru and later kills him by cutting his head off.
There's a HUGE love triangle in Kojima Akira's "Wa!" that it's no longer a triangle, but a circle. See for yourself:
In Area no Kishi, it is revealed in a flash back that the protagonist Kakeru realized that he had a crush on childhood friend and football teammate Seven. After working up the nerve to confess his feelings, he happens upon Seven as she is confessing her feelings for his older brother, Suguru.
In Durarara!!, Masaomi claims that there is a "love triangle" between Mikado, Anri, and himself. It's... more or less true, though it's mostly played for laughs than really anything serious. Besides, there's Saki to consider...
On a side note, there's also Namie -> Seiji <- Mika and Haruna -> Nasujima-sensei -> Anri.
In Yu-Gi-Oh! GX, Yubel loves Juudai and insists that Johan has stolen Juudai's love away from him/her. Johan and Juudai have a metric fuckton of chemistry together. And once Juudai regains his memories of his past, he loves Yubel. And in a sense did when he was a child, though not (yet) in the romantic sense, since he sent her/him into space to try to heal her/him of her insanity.
In Shugo Chara! the sub-plot seems to be Amu's love triangle. Ikuto and Tadase are both in love with Amu, but Amu loves Tadase. In the manga Amu says that she fell in love with his outer-character. Amu also seems to develop feelings for Ikuto as the story continues. Only Ikuto and Tadase kind of hate each other.
In Cherry Juice, Minami begins falling for his step-sister Otome, after she finally gets up the nerve to ask out his best friend Amane who she has been crushing on for years. As Minami becomes increasingly more jealous of their relationship, Otome starts to realize that she has the same feelings for him (which has been obvious to her friends and grandmother for a long time.)
In Future GPX Cyber Formula, Karl Lichter von Randoll wants Asuka Sugo, who's in a relationship with Hayato Kazami, and at one time (in which Hayato and Asuka's love hasn't developed until towards the end of the TV series), Randoll made a bet that if he wins the Spanish GP, he gets Asuka's kiss. Thankfully, he didn't.
The main love triangle in Star Driver involves the main protagonist Takuto, the heroine Wako, and Wako's arranged fiancé and childhood friend Sugata. Sugata is devoted to Wako but doesn't seem to mind the fact that Takuto and Wako are gradually becoming closer, Takuto and Wako show an interest in each other in spite of the engagement circumstances, and Wako seems to harbor strong feelings for Sugata than one would predict (and she is a Yaoi Fangirl for Takuto/Sugata) — make of what you will with that. Nevertheless, the love triangle is even lampshaded in an episode, where Takuto wonders if they were all really friends or if he was only a "third wheel" to a "couple."
The final episode has Wako confirm that she's in love with both - and that's its difficult to choose between the two of them; so much she briefly wishes that Takuto had never come.
In the second half of GUN×SWORD, Van is surrounded by an unwanted harem, with Pricilla and Wendy making a triangle as the main competitors for his interest. But the most significant triangle for him is the one involving himself, his dead fiancee Elena, and his most important love interest: Wendy. Meanwhile, Wendy is also involved in a triangle with Joshua, whom Yukiko seems to like too. It's not clear whether Wendy even realizes that Joshua has a crush on her, though.
There are currently three love triangles in Kimi ni Todoke: Sawako/Kazehaya/Kurumi, Ryu/Chizuru/Tooru, and Kazehaya/Sawako/Kent. All 'Bettys' win in the end.
This does get resolved... in favor of Tamaki, on the final chapters of the manga... Although the Anime version left it ambiguous...
In Xam'd: Lost Memories, there's the hero Akiyuki, his best friend Furuichi, and their mutual love interest Haru.
Zero, Yuuki, and Kaname from Vampire Knight. Sooooorrta resolved in the end. Kaname ends up in a coma, Yuuki marries Zero and they raise their own child as well as Yuuki and Kaname's kid, Zero dies of natural causes, and once everything's resolved Yuuki sacrifices herself to revive Kaname.
There are a few standard love triangles in Onidere, but a hilarious one occurs between Tadashii (the boy), Saya (his girlfriend), and "Angie" (Saya again, wearing a Paper-Thin Disguise). To add to the hilarity, Tadashii sees through Saya's disguise immediately, and is so incredibly confused.
Cantarella has a perfect love triangle, illustrated here:
Tenshi Ni Narumon has, for majority of the series, something akin to 'love quindrangle' Noelle-Yuusuke-Natsumi-Fuyuki/Raphael-Mikael-Noelle, but it turns out Mikael never was interested in Noelle in that way (he is in fact in love with Raphael), so here the chain breaks. And poor Natsumi is the one left out..., as well as more traditional triangles Muse-Eros-Silky-Noelle, kinda tragical, because Muse and Eros are just dolls and Silky expresses weird fixation/hatred towards Noelle. Also, Miruru-Gabriel-Sara-Mikael - where the one with poor luck is Sara.
In Gankutsuou, there are two major love triangles, in two different generations. Edmund Dantes and Fernando Mondigo were both in love with Mercedes, who was engaged to Edmund. Fernand gets proactive. In the next generation, Franz and Eugenie are both in love with Albert, who is engaged to Eugenie. Franz had long ago made the decision to supportAlbert and Eugenie, and dies without Eugenie or Albert ever knowing that there was a love triangle.
Then there's a more minor love triangle, when Maximilian and Franz's fiancee, Valentine, fall in love. Decidedly resolved when Franz pledges his support to Maximilian and helps him rescue Valentine from her family. The most easily resolved triangle, what with the hypotenuse being a gayShipper on Deck.
More of a love square, but in the background, both Jet and Droy are trying to win Levy's heart. However, most Ship Tease with her has been with Gajeel instead, and yet the two of them seem to be Just Friends, if that. Not that it stops Jet and Droy from hating him that much more.
Many fans saw Lucy and Lisanna vying for Natsu as a love-triangle, but since Lisanna's screen-time with Natsu disappeared after the Edolas arc in the manga, it pretty much seals Lucy's victory. The anime producers (who have been proven to be huge Lisanna fans) say otherwise as they try to squeeze in moments between Natsu and Lisanna in the anime filler such as what's seen in episode 163 of the Magic Games arc. Afterwards, the Tartaros arc in the manga finally dedicates some actual screen-time between Natsu and Lisanna starting from chapter 367; reigniting the Love Triangle.
There's also a very odd one in regarding Gray and Lyon for Juvia. Juvia loves Gray, but Gray doesn't show romantic interest. Lyon likes Juvia, but Juvia finds him creepy. Lyon sees Gray as a love rival, but Gray doesn't understand.
Tokyo Crazy Paradise has one with Akira, Ryuji, and Tsukasa. Possibly with Asago, Tsukasa, and Ryuji.
True Tears has a confusing and somewhat depressing web of love. The main character, Shinichirō is in love with his childhood friend Hirome, who is also in love with him, but because she thinks they're half siblings acts coldly towards him and pretends to be in love with a rival basketball player, Jun, who is actually in love with his own sister, Noe, who is in love with Shinichirō. Jun and Shinichirō then agree that Jun will go out with Hirome if Shinichirō will go out with Noe, as they believe that that is what the one they love wants. Meanwhile, Shinichirō's friends Aiko and Miyokichi are dating, but Aiko has feelings for Shinichirō and Miyokichi doesn't know. Hilarity Ensues.
Rinne: The two main triangles are between Rinne, Sakura, Tsubasa and Rinne, Ageha, Sakura. Rinne and Tsubasa like Sakura (Sakura's feelings are unclear at this point) and Ageha crushes on Rinne who only tolerates her.
In ½ Prince Lan/Prince has, in addition to a swarm of female admirers, two main love interests, Zhou/Wicked, who knows she's a girl, and Gui, who thinks she's a boy but states that he doesn't really care about her gender. While she's initially annoyed with Gui and oblivious to Wicked's feelings, she eventually realizes that both of them really love her, and begins to angst over how she'll have to seriously hurt either her childhood friend or the man who's cheerfully sacrificed for her in subtle ways since she met him. She ends up married to the latter, Gui.
In Haruhi Suzumiya's student film, "The Adventures of Mikuru Asahina", there is some sort of love triangle between Mikuru, Yuki Nagato, and Itsuki Koizumi, to the point where the two girls transfer into Itsuki's school. However, this was just a case of Haruhi losing sight of the characters' actual conception: Mikuru is a Magical Girl from the future, Yuki is a Human AlienDark Magical Girl, and Itsuki is the Living MacGuffin whom the former two battle over.
Kyon: Yuki and Mikuru's story of battling has now turned into a heated romantic comedy, or rather nothing more than a love triangle.
Gakuen Babysitters has a love triangle involving Ryuuichi, Ushimaru, and Inomata. However, it's not really a major element of the story.
Tamako Market has a bisexual one between Mochizou (male) and Midori (female) towards Tamako, particularly fleshed out in episode 5.
Two triangles develop around Judy in the last third of My Daddy Long Legs. First, there's the Judy-Jimmie-Julia triangle. Then there's the more significant Jimmie-Judy-Jervis triangle. Judy chooses Jervis; she never seems to have had romantic feelings towards Jimmie at all. (This is how it was in the books too)
An interesting variation in Yurara. Yako and Mei are both in love with Yurara. However, Yurara, thanks to her Guardian Spirit, can transform into a superpowered alter ego, with a slightly different personality. And it so happens that Mei is in love with "regular" Yurara, who is falling for him in return, while Yako has feelings for "Guardian" Yurara, who can't stand Mei and loves Yako instead.
In the pilot episode, there was one between the childhood friends Genzou Wakabayashi, Tarou Tsubasa and Aki Yamazaki. Genzou asked Aki out, but she politely refused him as she already liked Tarou.
In the series itself, the first part's potential love triangle between Tsubasa Ohzora, Sanae Nakazawa and Yayoi Aoba fizzled out almost immediately after Yayoi's first appearance, as she focused more on her team captain Misugi. However, in the second part the trope was played straight between Tsubasa, Sanae and the new assistant Kumi. Sanae won easily, and so much that she managed to marry Tsubasa when they were old enough for it.
In HeartCatch Pretty Cure!, Tsubomi and Erika help out inspiring mangaka Ban with an love triangle... in his doujinshi about the Cures - in his story, Blossom had fallen in love with a handsome man (who looks like a fusion of Ban and Itsuki) and Marine has opted to help her. However, she ends up falling in love with him as well. It's this situation that puts him at Writer's Block. Tsubomi and Erika act out the perfect ending - the two girls confront each other and decide that their friendship and duty is more important than one guy.
Deconstructed in the anime version of School Days, which appears at the beginning to be a light-hearted High School romantic comedy. Then Boy (possibly) gets Girl A (who he is dating) pregnant, which causes him to become an absolute Jerk Ass and start sleeping around. Girl A murders Boy, then Girl B (who Boy was dating but never got around to breaking up with, stringing her alone as he dates Girl A and Girl B gets atrociosuly broken) finds the body and confronts and murders Girl A. Girl B then decapitates the Boy's corpse, takes the head and relaxes on a nice boat with it. However, the original Visual Novelcan play this very differently...
And then there's Jean/Scott/Emma...even though "Jean's dead again", that doesn't stop the writers.
The younger generation isn't free from this either. Young Avengers has Kate Bishop in a Betty and Veronica situation with Patriot (Betty) and Speed (Veronica), and Stature is in a tweak of the Superman variation with Iron Lad and the Vision, whose brain is patterned off Iron Lad's memory engrams.
The only stable couple? Wiccan and Hulkling, the resident Yaoi Guys.
Happens in the same fashion to Hal Jordan. At about the same time that Hal received his ring, Carol Ferris broke up with him in order to focus on running her father's business. That didn't stop her from chasing after Green Lantern whenever she got the opportunity.
Different adaptations of the Superman mythos (i.e. the 90s' Lois and Clark TV series) will throw in Lex Luthor as a competitor for Lois' affections. Results may vary.
Superman's ex-girlfriend Lana Lang is also in competition with Lois for his affections. In some versions of the story, Lana is just as unaware as Lois that Clark is Superman. In other versions, Lana has known since high school, and it was the initial reason they broke up—resulting in her rocky marriage to Clark's friend Pete Ross, making this a Love Dodecahedron. Superman himself long carried a torch for his college girlfriend Lori Lemaris, and was bitterly disappointed when she rejected him for Ronal, whom she married (Lori and Ronal are both merpeople, incidentally). Superman and Lori are now "Just Friends.".
Does Clarabelle Cow go out with Goofy or Horace Horsecollar? Some of the comics involving them play with this trope.
Subverted in the erotic furry comic, Omaha the Cat Dancer. In this series, Omaha runs away from her lover, Charles Tabey Jr., in utter desolation after he blew up on her for a dark secret she had to confess (She was already married, had broken up with her husband long ago, but never actually divorced yet) and goes to a small town where she starts a passionate relationship with another man. However, Omaha learns that Charles was hospitalized with a nervous breakdown because her leaving him and she decides to return to him. Before she does that, she explains the full situation with her new boyfriend and he is understanding enough to encourage her to go back. After all differences are patched up, Charles makes a point of inviting that man to their New Year Eve's party to show his appreciation for helping Omaha at that tough time he himself put her through. As it happens, the pair get along so well they agree to take Omaha to bed a threesome and a fun time is had by all.
Empowered/Thugboy/Ninjette. Type 4, or a type 6 if you have your yuri goggles on.
German artist Walter Moers once wrote such a story, involving a (male) fir tree, a (female) triangle (as in the instrument; forms a pair with the tree) and a toblerone bar (male, bisexual) who cheats with the triangle. While they're in a tent. (Triangle / triangular story, get it?) As the author Lamp Shades: Doesn't that sound pretty constructed?
Sin City is normally not the sort of place you might expect a love triangle, but we have at least two:
Dwight bounces back and forth between Gail, the badass prostitute and Shellie, the sweet waitress.
Then we have an All Love Is Unrequited situation. Marv loves Nancy, who loves Hartigan, who only loves her in a platonic manner. It's a rare example in that Marv and Hartigan never knew each other.
ElfQuest has a few, complicated by Recognition. This is a "mating urge" bringing together a pair that are a good genetic match to produce gifted children, and traditionally a Recognised couple will become lifemates.
Leetah's choice between Rayek and Cutter.
Dewshine, Scouter, and Tyldak (when Dewshine and Tyldak recognised, but couldn't stand each other)
Dewshine, Scouter, and Tyleet, when Scouter and Tyleet recognised, ending in a stable threesome
Ember's choice between Mender and Teir.
In Pocket God, Nooby has a crush on Sun, but get jealous when she starts hanging out with Ooga. Nooby gets over it by issue 21, when he realizes they still care for him.
In Spy Boy, Alex has a liking to both Bombshell and Spy Girl.
In the Tamers Forever Series one exists between Takeru, Kari and Pauline. You'd be surprised by how many people are Takeru/Pauline shippers.
Love triangles are a favorite fanfic trope, especially with an OC coming between the two leads.
On the BLU team, Sniper and Spy vie for Scout's affections in the Team Fortress 2 fic Paternal.Sniper calls it quits after realizing he's too old for Scout and Spy wins, though the three remain friends.
Jared↔Lyla Jared↔Rachel, from a flashback in chapter 9. Jared and Lyla almost married, but on the wedding day, Lyla found the sexual affair between Jared and Rachel. Lyla ran away from the wedding, and decided that she no longer wanted the love of any man.
Lyla←Elhael Lyla←Glorfindel. Both male elves want Lyla, and Lyla likes both. Elhael and Glorfindel become rivals as they compete for access to Lyla. There is also confusion over whether each edge is a romance or just a friendship.
This is a major plot point in MLP fic Hands — Andrew and Twilight Sparkle end up in a relationship together, but an awkward situation with some pheromone-emitting flowers causes a mutual attraction between him and Fluttershy that only strengthens over time. One offscreen cat fight and a lot of awkwardness later, and the three of them ultimately agree to give a polygamous relationship a shot. And all things considered, it seems to be working out pretty well. And now Chrysalis is trying to get in on the act.
Bend It Like Beckham: Jules adores Joe who only has eyes for Jess. More triangular in that multiple supporting characters mistake Jess and the boyish Jules for a couple.
Flying Down to Rio: Julio is engaged to Belinha, and they love each other. Unfortunately, Belinha falls for Roger, who adores her back
In Top Hat: Jerry loves Dale, who also has Alberto madly in love with her. Dale believes that Jerry is the husband of her best friend, but slowly finds herself falling for him.
Casablanca is a classic, with Victor loving Ilsa who still loves Rick but whose duty lies with Victor...
In Pearl Harbor, we have Evelyn torn between her boyfriend Rafe, who was believed to be dead, and his best friend Danny, who befriended her after Rafe was reported to be dead.
The Chinese film Painted Skin is just one big love triangle pileup. With certain supernatural elements. But no, it's mostly a bunch of love triangles (pick a main character, any main character).
Anchors Aweigh had the triangle between Clarence, Susan, and Joe. Clarence eventually finds another girl, and Joe and Susan end up together.
Sweeney Todd: The Demon Barber of Fleet Street: Johanna is loved by Anthony Hope and Judge Turpin (though the latter is more lust than anything else). Will she chose the beautiful young man who might help her break free - or the creepy old man who has the power to hurt her if she defies him?
In The Quiet Earth between Api, Joanne and Zac. They are the only known people left, so it's not surprising.
In TRON between main character Flynn, ex-girlfriend Lora and her current boyfriend Alan, writer of the eponymous program. This carries over into the computer world with Alan and Lora's program expies, Tron and Yori (with whom Flynn shares a Last Kiss).
In Last Vegas, Billy and Paddy both loved Sophie. She ended up marrying Paddy after initially choosing Billy, who told her that she belonged with Paddy. A year after her death, when the movie takes place, Billy and Paddy again end up falling for the same woman, and again, Billy tries to be selfless, but this time, Paddy won't let him and insists that Billy pursue his own happiness.
In X-Men: First Class, Raven and Hank hit it off almost instantly, but the moment he refuses to accept his mutant form (and her mutant form) drives her to attempt to seduce Magneto, who prefers her natural appearance just the way it is.
The "Hunger Games" movies play up the barely-even-there-at-all love triangle from the books into a much bigger thing, presumably to cash in on the "Twilight" fad. The first movie isn't so bad but the second manages to make it seem like Katniss is in a relationship with the guy she never has romantic feelings for in the book, while her relationship with the other boy is very downplayed. Suzanne Collins, who wrote the books, has stated that she didn't plan a love triangle of any kind but her publisher insisted that she add one.
In Last Vegas, when they were children, Billy and Paddy were both in love with Sophie. Now, in their 70's, they've both fallen for Diana.
The Kane Chronicles feature Sadie, having to pick between Walt and Anubis. Anubis is a Egyptian boy-god that the other gods forbid Sadie to go near and Walt is dying because of a curse. The triangle is solved by Walt hosting Anubis, thereby keeping Walt alive and letting Sadie near Anubis, as he's not in his godly form. Yes, the triangle is solved by the two love interests sharing the same body.
And then the whole problem caused by the triangle between Achilles, Breisis, and Agamemnon.
One Arthur C. Clarke story has a love triangle between some minor characters (due to the relationship-stretching difficulties of space travel) which all the participants are completely aware of. It's also "completely equilateral", if you get Sir Arthur's meaning. This editor would provide more information but he can't even remember whether it was a short story or a novel.
Rendezvous with Rama has several triangles, including one equilateral, although only the two men of that triangle appear in the story.
The Wheel of Time has all of Elayne, Min, Aviendha, and Rand's angsting before they settled on a polyamorous relationship. There was also Berelain's determined pursuit of Perrin, which utterly trashed his relationship with Faile, and the whole complicated set of customs and rules that put Bain, Chiad, and Gaul in their position.
A Midsummer Night's Dream, anyone? Helena loves Demetrius who loves Hermia who loves Lysander, who loves her back. And that's before the fairies start mucking around with their relationships.
It's resolved early on when Hermia decides to leave Athens with Lysander secretly.
Then Helena, who's a Yandere by the way, decides to tell Demetrius of this, who, by the way. loves Hermia and hates Helena. Hilarity Ensues.
Warrior Cats loves these. Spottedleaf and Firestar liked each other in the beginning, but she died and promptly began stalking him from above. He then fell in love with Sandstorm, although for a while she wasn't so sure he wasn't still in love with Spottedleaf. And he somehow managed not to realize that he and Cinderpelt weren't Just Friends.
Then, the sequel series has two more. Brambleclaw and Squirrelflight love each other, but they fall out for a while and Squirrelflight hooks up with Ashfur. They make up in the end and become mates, but Ashfur never forgives her, causing him to go insane, betray the Clan, and try to kill her children—forcing her to admit that they aren't hers. Brambleclaw, who had no idea, left her because of this. As of now, their relationship stands on uncertain ground. Stormfur was also sweet on Squirrelflight for awhile, but later fell for Brook.
And then there's the infamous Crowfeather/Leafpool/Feathertail/Nightcloud triangle. Crowfeather fell for Feathertail, who then promptly died, and later ran off with Leafpool because they could never be together because of the boundaries between their Clans, but eventually returned. To prove his loyalty, he hooked up with a cat from his own Clan, Nightcloud, and a kit with her. It was later revealed that Leafpool actually had Crowfeather's kits and was forced to give them up to her sister at first or risk punishment. When this was revealed, he rejected her completely and declared that his mate was Nightcloud and his kit was Breezepelt.
Lionblaze used to love Heathertail, but gave her up (much to her dismay) because they were from separate Clans and he couldn't be a great warrior if he was seeing a cat from another Clan. They grew to hate each other, and now Heathertail and Breezepelt may or may not be Just Friends.
In Les Misérables, Eponine loves Marius, who loves Cosette, who (eventually) loves him back after a bit of trepidation.
Jack Campbell's The Lost Fleet at least for the first four books. While its clear that Desjani is head over heels for John Geary, Geary is already in a relationship with Rione, which gets complicated when they find out Rione's husband may not be dead. And they're all on the same ship.
In the Darkest Powers series, Chloe has feelings for both Simon and Derek, and both of them have feelings for her. Simon and Derek are brothers. Awkward. In a refreshing change from most female protagonists who find themselves in this situation, Chloe doesn't toy with either of them. She's also honestly confused about her own feelings, and barely realizes that she has more-than-friendly feelings for Derek. In the end, it turns out that she does like Simon, but just as a friend. She's honestly in love with Derek, even if it takes both of them a bit of time to work things out and get together. Some completely understandable angst ensues between Simon realizing that Chloe likes Derek, and Chloe and Derek finally figuring out that their feelings are mutual, but everyone involved is actually mature about it, completely free of pettiness, and no one resents anyone because they all act like rational people.
In the beginning, it looked like the triangle would be a rather generic Main Character Girl vs. Alpha Bitch for The Nice Guy between Chloe, Simon, and Tori, though this was resolved fairly early when Tori’s mother warned her to stay away from Simon practically under threat of death. Tori then tells Chloe that Simon is all hers right before hitting her in the head with a brick and hog-tying her as revenge for making her already crappy life even MORE miserable. Though, she gets over her crush after being forced to spend a few days alone with him. And then later on it turns out they’re half-siblings, which explains why Mrs. Enright was so adamant about Tori staying away from him. Cue awkward music!
Also played with in The Reckoning when Tori makes a comment about Derek actually not looking so bad, though this was more a casual observation and a chance to tease Chloe than anything serious. Doesn’t stop Chloe from getting mad, though.
In John C. Wright's The Golden Age, The Phoenix Exultant, and The Golden Transcedence, Phaethon is in love with his wife Daphne Prime, who drowned herself in a simulated dream, and emancipated a partial personality, Daphne Tercuis, whom she thought would be better suited to Phaethon, and who is madly in love with him. Owing to Laser-Guided Amnesia, neither Phaethon nor Daphne Tercuis know this at the opening, and because Daphne Tercius derives from Daphne Prime, she had most of her memories, and Phaethon confuses the two of them. Eventually he realizes that most of his fondest memories of Daphne are of Daphne Tercius and asks her to marry him in her own right.
In the Brazilian book The Devil to Pay in the Backlands, Otacília loves Riobaldo, who likes her and could even love her back if he wasn't already in love with Diadorim, who seems to love him too, but he has his own reasons - besides being both male - to stay quiet.
The Mortal Instruments: Clary falls for Jace. Long time best friend Simon has always loved Clary, but Clary doesn't find out until later. Alec has a crush on Jace, who doesn't find out at all. Simon has a crush on Isabelle, Alec's sister. She loathes him when they first meet but he grows on her. By the end of the first book, Jace has fallen for Clary as well. Oops. Later, Maia is introduced. She likes Simon, and he likes her back. But we can't let anything in this story be simple. Sorry Simon.. By now, Magnus Bane has been introduced. Guess who ends up with him? In City of Glass, yet another player is introduced, Sebastian. Clary has a short fling with him. Oops again. Only it turns out that Jace and Clary aren't actually related. But in Sebastian aka Jonathan's case, it's for real. And he knew it. And this is to say nothing of Jocelyn's love history, which is simpler (Lucian / Valentine), or other side love interests like Isabelle's fling with Meliorn or Maia's ex Jordan. As of the fifth book,Clary/Jace is on with difficulties, Alec/Magnus is off, Jocelyn/Lucian is on, and Maia/Jordan is on.
Sisterhood Series by Fern Michaels: By the book Deja Vu, one has clearly developed between Maggie Spritzer, Ted Robinson, and Abner Tookus. Maggie loves them both, but in different ways. She was engaged to be married to Ted. However, she calls off the engagement to Ted and says that the two should just be friends. She tells Abner how she feels about him, but he does not forgive her mistreatment of him, and their relationship comes to an end. The book Home Free has her hooking up with Augustus "Gus" Sullivan, and apparently Abner finally forgives Maggie. Whew!
Taylor Anderson plays with love triangles as sub-plots in his "Destroyermen" series.
The first one introduced involving Chack, Selass, and Saak-Fas (eventually a fourth party, Safir, gets thrown into the mix). Chack initially loves Selass, but Selass puts him down and chooses Saak-Fas, another suitor. After Saak-Fas is lost in battle, Selass refocuses her love on Chack. Oddly enough, this same battle is what makes Chack forget about Selass and when Saak-Fas is found alive as a prisoner of war/rations for the Grik, Selass is unable to mate with Chack for a while. Once Chack meets Safir, Selass becomes the unlucky suitor instead of Chack.
Is somewhat downplayed and possibly subverted with one of the human Destroyermen, Dennis Silva. Silva attracts the attention of the human nurse Pam, and Lemurian warrior Risa. His solution to this is that he has relationships with BOTH of them simultaneously, as he loves them equally, and both females seem to consent with this.
In Poul Anderson's Time Patrol story "Brave to be a King", Manse had lost out in one in the Back Story. When Cynthia comes to him to help save her husband, Manse wrestles with the temptation to follow the rules easily — at Denison's expense — because of this. At the end, Denison realizes that his years in the past with Cassadane meant more to him that his brief marriage with Cynthia, even though he had chosen Cassadane because she reminded him of Cynthia.
Rick Riordan loves this trope. First there was Percy Jackson and the Olympians, where hero-turned-villain Luke was implied to have had feelings for the seemingly deceased Thalia, and was fancied by Annabeth, who also slowly developed a conflicting crush on main protagonist Percy, who returned the crush but also had mixed feelings for Rachel, who was attracted to Percy but in the end figured out that she was attracted to him jut because he was her key to the mythological world, while Annabeth figured out that she loved Luke as a brother, conveniently after the latter had performed a Heroic Sacrifice.
The sequel, The Heroes of Olympus, has Piper, who likes amnesiac Jason, who also likes her but is conflicted because he has just met Piper and he vaguely remembers another girl from home, named Reyna, who is implied to have feelings for him but needs someone to replace him with in the role of praetor, and promptly asks Percy, also informing him that the position would require a certain closeness, while Annabeth, Percy's girlfriend, searches for him all over the country. In the meantime, Frank gets involved with Hazel, who had a boyfriend back in the 1940's who appears to be related by blood to present-day Leo, since they look exactly the same.
Before the series's start (this is recounted in flashbacks), Catelyn Tully was in love with Brandon Stark, while her father's foster son Petyr "Littlefinger" Baelish loved Catelyn (additionally, Catelyn's younger sister Lysa was in love with Petyr but he never reciprocated her feelings). So when Brandon came to claim Catelyn's hand, Petyr dared the much older, vastly stronger and vastly more skilled Brandon to a sword fight over Catelyn. Petyr was nearly slain and developed a seriously unhealthy obsession with Catelyn (who, by the way, had to marry Brandon's brother Eddard after Brandon died). In A Game of Thrones, Petyr manipulates events to contrive ways for him and Catelyn together. When he fails, he goes after Catelyn's young teenage daughter Sansa..
And there's another major love triangle before the series' start: Robert Baratheon/Lyanna Stark/Rhaegar Targaryen. Robert was in love with Lyanna and betrothed to her. She was abducted by Rhaegar Targaryen, who had named Lyanna his "Queen of Love and Beauty" at an earlier tourney despite his marriage to Elia Martell. It's implied that Lyanna's abduction was the final straw for Robert, who started a war against the ruling Targaryen dynasty and personally killed Rhaegar during one of the battles.
A third: Robert Baratheon/Cersei Lannister/Jaime Lannister. Cersei and Jaime had been having sex since they were twelve, but it stopped when Cersei was betrothed to Robert Baratheon. Despite her original infatuation with Rhaegar Targaryen, Cersei was the happiest woman in the Seven Kingdoms marrying Robert, but Robert was still too heartbroken over the death of his original betrothed (Lyanna Stark), causing Cersei to seek solace in rekindling the relationship with her brother. As a result, all three of Robert and Cersei's children are actually fathered by Jaime.
The School For Good And Evil features a triangle between Sophie, Agatha, and Tedros (son of King Arthur). Sophie loves Tedros, and Tedros loves Sophie, but in all of their lessons on picking Good from Evil, keeps picking Agatha. Tedros eventually comes to feel love for Agatha, who starts to feel something for him, but is loyal to her friend Sophie and keeps trying to mend their relationship. Sophie ends up a full-fledged witch, and then goes to various lengths to win/kill Tedros AND Agatha. Sophie eventually takes a death blow meant for Agatha, and then makes a Dying Declaration of Love to her ( She got better )
In Stephanie Burgis's A Tangle of Magicks , Kat realizes with a shock — as Lady Fortherington complains to Papa about how he had refused to listen to her warnings about Kat's mother— that she was jealous, having lost out in a triangle.
The First Shop Of Coffee Prince: Yoo Joo once left Han Seong for DK. She returns to Han Seong, but has a borderline flirtatious friendship with Han Kyul, who likes both her and Eun Chan. Han Seong also has romantic feelings for Eun Chan, but her feelings for him seem to be platonic.
Grey's Anatomy: Dr. McDreamy, Meredith, and Dr. McDreamy's wife. An interesting example, in that the audience is primarily rooting for the "interloper."
Practically every show on The WB and UPN that had more than a handful of episodes had at least gone through a long Story Arc centered on a triangle, including, oddly enough, the Professional Wrestling franchise.
During season 3, Willow is torn between Oz and Xander, and Xander is torn between Cordelia and Willow.
A short-lived triangle existed between Oz, Willow and Veruca in season 4.
Third Watch: Bobby Caffey is in love with his colleague Kim Zambrano... who is still in love with her ex-husband Jimmy Doherty.
Merlin [BBC] has, as in the legends, the Arthur/Gwen/Lancelot love triangle.
It's somewhat deconstructed considering Guinevere is legitimately in love with Arthur, and her tryst with Lancelot brought on due to an enchantment.
Earlier on, however, Arthur was falling in love with Guinevere at the same time she was falling for Lancelot, until Lancelot vanished from the face of the earth.
The third season of Degrassi The Next Generation has a truly grim Love Triangle that ends (after many, many twists) with all three people hating each other and vowing to never see each other again. Later seasons overused the Love Triangle to the point of insanity; an absolute majority of the cast has been in at least one Love Triangle, and some of them have been in two or more.
The aforementioned lovetriangle essentially recreates a love triangle from Degrassi High that was just as catastrophic. It will forever be remembered not only for breaking up what was perhaps Degrassi's most popular couple ever, but also for being the first time the word "Fuck" was uttered on Canadian TV.
LOST: Much of the drama of the series involves Kate trying to figure out whether she loves Jack or Sawyer.
Further muddled by the inclusion of Juliet, an extra love interest for Jack who gets stuck on the Island with Sawyer after Jack and Kate leave and the Island moves. Finally settled in the Grand Finale where the pairings settle as Jack/Kate and Sawyer/Juliet.
Jeff and Marty over Jeannie in Randall and Hopkirk (Deceased) (both versions), Marty has one advantage of being Jeannie's husband (or fiancé in the newer series) but one huge disadvantage in being dead and invisible to Jeannie.
Sophie, Paul, and Megan in the third season of Slings and Arrows are a textbook love triangle. In the show's backstory, Oliver, Ellen, and Geoffrey have a more complicated (type 12) relationship.
Stingray (1964): Atlanta Shore is in love with Troy Tempest, but in the pilot episode Troy meets and goes all dewey-eyed over Cute Mute Marina. This is such a key part of the show that it's even lampshaded in the closing titles.
The second season also had Ivanova-Talia-Garibaldi in a rare Bi the Way variant.
The Closer: Agent Howard is in love with Deputy Chief Brenda Johnson, who's in love with Agent Howard... now. But back in DC Brenda had an affair with her superior, Chief Pope. Ironically, she's now working for Chief Pope again, this time in L.A., and he still has feelings for her, but she's over him and married to Agent Howard, resulting in a very grudging I Want My Beloved to Be Happy.
On Angel, Fred is pursued by both Gunn and Wesley during Season Three. Gunn wins at first, but Wesley eventually gets with her, a year after Fred and Gunn break up and just before Fred's death.
There's also the Angel-Cordelia-Connor love triangle in Season Four, complicated by the fact that Angel and Connor are father and son and that Cordelia is possessed by the Big Bad.
Glee has love triangles for the kids AND the adults. The kids are Finn, Rachel, and Quinn while the adults are Will, Terri (Will's wife), and Emma.
The Will/Terri/Emma triangle has been (mostly) resolved because Will found out Terri was faking her pregnancy and promptly divorced her. The Finn/Quinn/Rachel triangle was resolved as well as Finn learned the baby wasn't his. The second triangle was replaced with a Rachel/Finn/Jesse triangle until Jesse was written out of the show.
In the season 1 of Heroes, the triangle of Peter, Simone and Isaac was epic and led to catastrophic consequences.
The Office (American version), in one of the most hilarious love triangles ever conceived, with Andy Bernard and Dwight Schrute competing for the affections of Angela Martin. It did lead to accusations of Soaperizing, however.
Triangles between Jim Halpert, Pam Beasley, and Roy Anderson... followed by Jim, Pam, and Karen. As well as at least one episode of Jan, Carol, and Michael and another of Andy, Dwight, and Erin. Andy, Dwight, Angela was hardly the first (or last with the newer Dwight, Angela, Isabelle triangle) time that show has used a love triangle.
The British version also highlighted the tension between Tim/Dawn/Lee, as well as Rachel/Tim/Dawn.
Battlestar Galactica has its own heaping helping. Starbuck is torn between Apollo, who marries Dee, and Sam Anders, while Tyrol used to be involved with Boomer, but marries Callie and has a son with her. Or so he thinks. Even Colonel Tigh gets in on the action, falling in love with a Six, but still has feelings for his not-actually-dead wife.
The original Galactica series had Athena-Starbuck-Cassiopia
Chuck: Chuck loves Sarah Sarah loves Chuck back but her ex boyfriend Bryce wants Sarah back and Sarah still cares for Bryce there are more to Sarah:Cole Chuck:Lou and later Jill.
Also, there was a love triangle between Adam/Cassie/Jake later on in the season.
The Vampire Diaries: Stefan and Damon compete for Elena (because she looks like Katherine, a woman whom they both used to be in love with).
There are lots of other triangles in the show as well, but most importantly as of the second season both of Elena's best friends also have love triangles of their own.
Gilmore Girls does this with Lorelai, Luke, and Christopher and Rory with Dean, Jess, and Logan.
Two notable examples on Farscape - John, Aeryn, and Gillina in the season one finale, and John, Aeryn, and John in season three. John Crichton is involved in a love triangle with himself.
Two examples in Doctor Who: First between the Doctor, Rose and Jack, and later between the Doctor, Amy and Rory.
Jack and Rose are both in love with the Doctor, and also fancy each other. The Doctor cares about Rose in his own way, but the show leaves it ambiguous whether it's deep friendship or romantic love, and the Doctor Cannot Spit It Out either way. All three pairings get sweet and emotional kissing scenes. The Doctor eventually solves the Rose side of the problem by convincing her to live Happily Ever After with his half-Donna clone, which is messed up on a whole lot of levels.
Amy and Rory are Happily Married, but Amy also quite fancies the Doctor. The Doctor wants nothing of that, pushes Amy and Rory together and freaks out when Amy tries to kiss him against his will. Just as well for Rory, since the Doctor literally can, and does, fly Amy to the moon and take her on a spin amongs the stars.
Amy turns out to be completely shameless during her and Rorys wedding.
Amy: [moves up to the Doctor] You absolutely definitely may kiss the bride.
The Doctor: [Casually puts his index-finger over Amys lips] From now on, I leave all of the kissing to the brand new Mister Pond!
Amy: Oi! Where are you off to? We haven't even had a snog in the shrubbery yet!
Amy: Shut up, it's my wedding.
There's another sort of love triangle between The Doctor, River his wife, and the TARDIS herself (Idris).
Alias included several love triangles; however, the most prominent were between Will/Sydney/Vaughn and Sydney/Vaughn/Lauren.
Fringe appears to be setting up a mind-frak of a love triangle between Peter, Olivia, and Alternate!Olivia.
The three-season long back and forth between Telema, Kwame, and Soji in Tinsel. Then there was also Angela/Reginald/Amaka, then Reginald/Amaka/Sunom before Sunom killed Reginald.
And in Season 5, Telema/Soji/Bimpe.
Beautifully (and possibly uniquely) done in an episode of Community; when Troy and Abed develop a simultaneous crush on the new librarian, their response is to approach her at the same time, explain the situation and invite her out on a mutual date so that she can determine which of the two she'd prefer to see again. She ends up settling on Troy, but he breaks it off when she says some unflattering things about Abed.
In the season 5 premiere, the sequel to the season 4 finale has Freddie kissing Sam and returning the feelings Sam has for him. However, that didn't last long though.
Miranda Cosgrove even hinted at some magazine interviews that there was a love triangle in the show.
The trope has become a game show - GSN's Love Triangle. Wendy Williams hosts the show, designed to chop one branch off the triangle after 30 minutes of somewhat intrusively personal questioning. The show debuted in the spring of 2011 for a first season of 36 episodes. Williams' hammy hosting is one of the many reasons nobody expects a second season.
In Frasier, Niles was in love with both Daphne (who was oblivious), and his unseen wife, Maris, finding himself torn for several years because he refused to cheat on or break up with his wife, yet couldn't get rid of his feelings for Daphne. Contrary to most triangles of this kind, however, Maris was so evil that fans spent all those years screaming "dump the bitch!" instead of rooting for Niles' marriage to succeed.
Downton Abbey has this in spades, with the most prominent example happening among the house staff in season 3 (Daisy fancies Alfred, who pursues Ivy, who likes Jimmy, who's hit on by Thomas).
Rookie Blue loves this one. Lets see: Sam/Andy/Luke, Dov/Gail/Chris, Andy/Luke/Jo, Traci/Jerry/Dex...that's every single one of the rookies in one at some point over two seasons.
In AMC's The Walking Dead, there is a type 4 between Shane Walsh/Lori Grimes/Rick Grimes — now that Lori's husband whom Shane and Lori thought was dead has returned and the affair with Shane ended.
The Amazing Race 2 had Wil/Tara/Alex. Wil and Tara were married but separated, but Tara still started flirting with Alex, and cost them the race by doing so.
Zach/Flo/Drew on Season 3. Zach and Flo were friends seeing if they could be more, but Flo fell for Drew instead.
There was a love triangle between Sara/Aitor/Lucas in Los Hombres De Paco. Sara and Lucas got married, then he went to the USA for CIA training (the series takes place in Spain) and then Sara had a long-term affair with Aitor while he was gone. She then got married to Aitor, while still be married to Lucas. Lucas came back home and then Sara went back to him.
Chinese Paladin: Ling'er loves Xiao Yao, and vice versa, but believes things will be better if she leaves him. Yue'Ru loves Xiao Yao, but knows it is unrequited. Both girls then attempt to bow out and let the other have a chance to be happy with the man they love, to the point where the man in question tells them both to shut up and stop helping him.
There is a lopsided one between Emma, Graham and Regina. It ends when Regina pulled a If I Can't Have You and crushed Graham's heart, killing him.
There was Snow White and Prince Charming loving each other despite the latter's Arranged Marriage to Princess Abigail, as well as their Storybrooke counterparts, Mary Margaret and David Nolan with his wife Kathryn. The former love triangle was resolved with Charming leaving his wedding for Snow and Abigail being reunited with her true love Frederick. The latter's triangle is resolved when Kathryn pulls an I Want My Beloved to Be Happy and everyone gets their memories back.
In season 2, Prince Phillip and Aurora are each other's true loves but Aurora suspects that Phillip's longtime friend and comrade Mulan might be in love with him too. His untimely death at the wraith leaves this unresolved. But then he's resurrected and it turns out Mulan has feelings for both of them and decided to leave them to save them all the pain.
In Season 3, there's one between Emma, Hook, and Neal. Neal is still in love with Emma and Hook has feelings for her. Emma herself is conflicted with her love for Neal and mixed feelings about Hook, so she shuts them both down and admits the only one that she has enough room in her life for is Henry, her son with Neal. In Episode 15, Neal dies, leaving most fans to speculate that Emma will end up with Hook by the end of the season.
In Season 3, one arises unexpectedly between Regina, Robin, and a time-traveling Marian.
In BBC’s Robin Hood, Guy of Gisborne and Robin Hood are both infatuated with Marian, who loves Robin but cannot have him because he’s an outlaw. Marian also is obligated to marry Gisborne in order to save King Richard’s life, and is genuinely on-again-off-again sympathetic towards him despite his villain status. The triangle gets resolved in the series 2 finale when Marian finally tells Gisborne that she loves Robin Hood and intends to marry him, and Gisborne kills her.
Appeared to be the case in ShougekiGouraigan, between the Protagonist Gou, his Evil Former Friend Jin, and their mutual love interest, Rin. Flashbacks initially seem to hint that Rin genuinely loves Gou and Jin is deliberately convincing himself he and Rin were in love, but when Rin returns it turns out she really is in love with Jin, sending Gou into a Heroic BSOD
As of season six of The Mentalist, we have Jane/Lisbon/Pike, with both guys vying for Lisbon's affection.
Salem: Mary Sibley and Anne Hale both have feelings for John Alden. It gets downplayed later in the season as Anne has her own witch powers to come to terms with.
Myth And Legend
The King Arthur stories largely revolve around the triangle of Arthur, Guenivere, and Lancelot. Interestingly, each of them originally came from a different story line, and merged together sometime in the 12th centuries. It's widely believed to be the French poet Chrétien de Troyes, who wrote about it in his Lancelot or The Knight of the Cart.
Defied in the song Au fond de temple saint from the opera The Pearl Fishers: when two close friends realise that they have fallen in love with the same person, they vow to renounce her so they do not become enemies.
The song "Withered Hope" by They Might Be Giants describes a love triangle between Withered Hope, her "Soul Mate", a crook, a motorbike, and Sad Sack (who loves Withered Hope).
Sarah Brightman has "Make Up My Heart". The singer has two great men in her life, but she wants one in her life. The whole song is dedicated to her talking about her dilemma and what one man has that the other doesn't.
Suede's "Asphalt World": "When you're there in her arms / And there in her legs / Well I'll be in her head".
The opening song for Macross Frontier, named Triangler sung by Maaya Sakamoto is a deconstruction of this trope. It tells of the feelings and crushing doubt that one who is in that situation might feel. An alternate version of this song is sung by Ranka and Sheryl and the version is aptly named Triangler Fight on Stage. Lyrics here.
Frontier also has the other song Tablet which is sung from the POV of Sheryl and Ranka; they are both asking Alto to pick the other because they want them to be happy and apologizing to the other for falling for him while also thanking each other for being a true friend. It is heartbreaking.
The real-life Edge-Lita-Matt Hardy love triangle (Matt and Amy "Lita" Dumas had been a couple both on-screen and off before she left him for Adam "Edge" Copeland, messing up his real-life marriage at the time in the process) crossed over into the wrestling world, temporarily getting Matt fired for complaining about it online and turning Edge into a seemingly unstoppable Karma Houdini.
The relationship between Eliza, Professor Higgings and Freddy in My Fair Lady is considered this.
Elisabeth of Elisabeth is the center of a love triangle with her husband the Emperor Franz Joseph and with Death himself, though her husband does not even know that der Tod exists. Death, however, is fully aware of the situation:
It is an old story, but for me, new Two have the same beloved — namely, you.
He tells Elisabeth at her wedding that she will regret the choice she has made because she loves him and not Franz Joseph, and he is right, despite Joseph's unconditional love for Elisabeth no matter how far she pulls away.
Played with in Skies of Arcadia. While Aika and Fina both show occasional feelings for Vyse, none of the three ever show any real desire to resolve the triangle. In fact, the two girls only seem to have an issue when Vyse shows interest in a third girl. Subsquently, some hints in the game drop that Vyse eventually becomes smitten with Fina, but he never acts on his feelings.
Drakengard has an interesting Love Triangle between Furiae, Inuart and Caim that's only alluded to as being serious one or two times in the whole game. And the sequel repeats the trend by setting up another triangle between Nowe, Manah and Eris. In the end, it is Manah who gets the guy.
There are several examples in War Craft III: The Stormrage brothers, Malfurion and Illidan, with Tyrande Whisperwind. Winner? Malfurion. Then Frozen Throne has a new triangle: Arthas-Jaina-Kael. Who wins? Probably nobody, since Arthas has completely thrown away his soul and love to become Lich King, and as of World of Warcraft, Kael is just thinking to have a hot sex with Kil'Jaeden.
But if you marry Bianca, you still feel like a jerk since the lavish wedding Flora's father had planned for his daughter becomes the lavish wedding for a stranger instead. Oh, and the remake adds a third bride, Deborah, and choosing her means dumping both Flora AND Bianca for a woman who just showed up.
The one that started it all was the Marth/Caeda/Catria triangle of Fire Emblem Akaneia, with Catria waiting in the wings, as Caeda and Marth have loved each other for years and get married at the end of Mystery of the Emblem.
The other two Pegasus Knight sisters in Marth's games are also in a love triangle like Catria's: the oldest sister, Palla, is in love with Abel (one of the two green/red cavaliers that the series always has); Abel ultimately falls for the youngest sister, Est, and they marry after the first game.
The Hardin/Princess Nyna/Camus triangle: Camus and Nyna love one another; Hardin has a huge Bodyguard Crush on Nyna, but while she cares for him as a person, she doesn't love him romantically... They get into an politically-based marriage, but Hardin eventually finds out that she loves another, so he becomes disenfranchised, and ultimately goes whacko when Gharnef gives him the Darksphere when Hardin's having a moment of weakness. Which includes capturing and then handing Nyna to Medeus as a Barrier Maiden. Bear in mind, Nyna marrying Hardin kicks off Mystery of the Emblem as a whole. This triangle itself ends messily - Camus winds up in Valentia after the events of the first game, withEasy Amnesia, and he falls in love with Teeta, a priestess and his caretaker. After Camus (under the moniker Sirius) frees Nyna from Medeus in Mystery of the Emblem, he mentions that "someone is waiting for him" in his home, implying that he's probably going to live with Teeta. Meanwhile, Nyna is alone and has decided to leave Akaneia in Marth's hands, as Hardin first was the Brainwashed and CrazyBig Bad and then died (and his Famous Last Words had him begging Marth to apologize to Nyna in his stead), and Camus wanting Nyna to be happy is implied to have backfired. Good going, Camus.
Priscilla adores Raven, who is partial to Lucius, who more or less returns his affections. Lucius is Raven's male companion and Priscilla is his little sister. Needless to say, Priscilla and Lucius's supports are brimming with unspoken hostility, which Priscilla recognizes and admits. Raven and Lucius have a shared ending... Raven and Priscilla do not. Then again, Priscilla has shared endings with three other guys (though only one can stay with her) and heavy ship tease with another.
Geo/Luna/Sonia, Mega Man Star Force. Funny that American fans tend to see more hints of Geo/Luna, while Japanese fans do the reverse.
Star Force 3 flirts with it. Depending on whose item you reclaim in the Stolen Items quest (Sonia's bag or Luna's handkerchief), one girl gets to hold hands with Geo while the other pigs out in an angry fit. If you pick Bud's shirt, both of them pig out.
Implied between Max, Grit and Nell in the first Advance Wars, before never being mentioned again. It was never one hundred per cent explained what happened, but Grit left Orange Star for Blue Moon to give Nell and Max the room they needed.
Subverted in Final Fantasy VIII. The game initially seems to be setting one up involving Squall, Rinoa, and Seifer, particularly when Rinoa thinks back to the time she spent with Seifer the previous summer and confesses that she thinks she might have been in love with him. However, by the time all three of them are brought together, Squall and Rinoa are beginning to grow close to one another while Seifer has fallen under the sorceress's influence, to the point that there's never any actual romantic conflict involving the three of them.
One exists in Jade Empire, between Silk Fox, Dawn Star, and a male player. You can exacerbate it by expressing an interest in both of them, resolve it by expressing an interest in only one, or if you have a high enough Charm skill you can have them both.
The first game has either human love interest, Liara, and Shepard, assuming Shepard led both of the others on enough.
The second game introduces some potentially complicated ones, as Shepard can have expressed an attraction to as many as five characters (either gender) over the course of the story.
The third game can potentially feature one straight-up if Shepard pursued a romance in both the first and second game, given that the developers explicitly described the ensuing situation as a "Love Triangle". Given the possibility of NEW romance options in the third game, this could quickly get out of hand.
Dragon Age also comes to mind. Origins has Leliana vs Morrigan if your characters male, and female players has Alistair vs Zevran, and Zevran and Leliana are bisexual so they're acually third options for the same sex player. Having two interested in the player makes one of your romances confront the player at some point down the line to force you to dump one of the romances.
Dragon Age II doesn't appear to follow the same format as Origins. No one seems to care if the player persues more then one romance. Not really surprising either since all romance options, excluding the chaste priest Sebastian, are bisexual. A Fenris/Hawke/Anders is the closest thing to being a love triangle. It's heavily implied that Anders has strong feelings for Hawke, even if he's turned down for someone else. Also Fenris continues to wear his alternate outfit (unlocked by romancing him) even if he's not the "endgame" romance and it's implied that he regrets running out. Fenris and Anders will have party banter about Hawke if she or he slept with Fenris and then with Anders after Fenris ran out after they spent the night together.
Neverwinter Nights: Hordes of the Underdark contains a severely underdeveloped one that might exist between Nathyrra, Aribeth, and a male player. It can potentially end in them accepting that you love them both. The sequel has a slightly better defined one (though Neeshka's romance plot was cut from the final game) between Neeshka, Elanee, and a male player and a rather obvious tension for female player's affections between Casavir and Bishop (whose romance plot fared slightly better than Neeshka's).
Vanguard Bandits as a few of these, with the Triangle being determined on which of the Multiple Branches the player decides to head unto.
There's an implied Triangle in Fallout 3 between Amata, the (male) Lone Wanderer and Sarah Lyons. The UST with Amata is fairly evident right from the beginning if you take the right conversation options while Lyons' only hint is a single line she drops at the end of Broken Steel where she expresses sorrow that if they would've met under different conditions, they might have "become... friends".
This being Fallout, you can screw it all up by damaging the Vault and forcing Amata outside, leading to her being interrogated and executed by an Enclave patrol as well as sending Lyons into Project Purity where she dies from radiation.
Fear Effect. Trailers for Fear Effect Inferno reveal that one would have developed between Hana, Rain, and Glas. Hana loves Rain very much, and she begins to show affection to Glas. Rain loves Hana very much, and she appears to want to have sex with Glas and maybe more than that. Glas begins to show affection to Hana, and he appears to be willing to have sex with Rain and maybe more than that. This would have been one complicated Love Triangle and how it would have been handled is not revealed. One trailer indicates that Rain and Deke have formed a relationship. Talk about complicated relationships!
Disgaea 4. Although it's never outright stated that Fenrich has a thing for Valvatorez, the game throws enough hints at you that it can be assumed he's in one of these with Vulcanus and Valvatorez. And when you consider his less that friendly relationship with Vulcanus, well...
In Mia's route in Duel Savior Destiny a love triangle of sorts that Taiga never seems to realize pops up between him, Lily and Mia. Lily never really has a shot, but Mia's jealousy leads to her becoming more unstable. The jealousy is somewhat understandable: Right as he confesses his undying love for her Mia is quite able to smell Lily's smell on him after the pair spent the previous evening having despair induced sex over the apparent loss of Taiga's sister and Lily's mother.
The Legend of Zelda has a few of these in some games, such as Kafei/Anju/Cremia in Majora's Mask and Pipit/Cawlin/Karane in Skyward Sword. Note that other alleged examples are only a result of the fandom's shipping, so the "triangle" between Midna/Link/Zelda does not count.
In Feel The Magic: XY/XX, you and another man are fighting a girl. In The Rub Rabbits!, you're going after one girl, and another girl is going after you.
Two in A Profile. Miou and Rizu love Masayuki, who used to date Miou until they broke up and starts out somewhat in love with Rizu. Messy, that. The other triangle is Kaine loving Miou loving Masayuki. Since both triangles together only contain four people, everything is settled once Masayuki starts dating any of his three options.
In Symphonic Rain there's Torta -> Chris <-> Ari from the start and in her route Asino -> Fal -> Chris, who may or may not reciprocate based on your choices.
Gets messy in Eien no Aselia. First, Kouin loves Kyouko loves Kouin, but Yuuto more than him. Yuuto is oblivious. Turns out surprisingly okay. Kotori also feels jealous of how close Kaori and Yuuto are since despite her rather blatant hinting he still hasn't caught on yet. This one doesn't end very happily until both Kaori and Kotori's memories get erased anyway though not exactly terribly, either.
The Tokimeki Memorial series is fond of these, as the protagonist usually has a rival for the affections of most of the potential love interests, and befriending both the intended date and the person who likes them will result in said rival becoming quite bitter towards the player (until things are smoothed over and the rival decides I Want My Beloved to Be Happy and tells you to take care of them). Tokimeki Memorial Girl's Side 3 actually allows the player to set up a love triangle with the datable guys, so that they'll fight over you instead.
Sakura and Nemu in Da Capo form the game's real triangle with their adamant refusal to accept Junichi pairing up with the other. Both say more or less that they'd be perfectly okay if he would pick any other girl, like Mako or Kotori or even some catgirl maid. Who... really does show up later.
Arcueid —> Shiki <— Ciel. Technically, there are 8-9 girls in love with him, but this is the set that has the triangle relation to it. In the more or less canon ending, Arcueid wins by default. Ciel's route makes Arcueid scary because she's losing/lost. Actually, Ciel and Akiha can also more or less make one of these as well, as, unlike the above, they genuinely hate each other. A lot.
Sayo, Itsuki, and Akira in the first chapter of Suika. In the second during the alternate route, Mie and Sayaka and in the third route, Akane, Touko and Yoshikazu. Only one of these ends well.
Canvas 2 has Yanagi, Kiri, and Hiroki. Yanagi loves Kiri loves Hiroki, who loves her back but pretends he doesn't to support his friend. Five years later, Elis and Kiri love Hiroki, who is completely oblivious and can pick from five other heroines instead.
In Higurashi no Naku Koro ni, though Mion is the only who showed clear romantic interest in Keiichi, there is a sort of minor love triangle between her, Keiichi, and Rena.
In the last episode of Rei, Rena confesses to Keiichi, though it may have been exaggerated by the magatama's power, it's still heavily implied that she loves him without the magatama.
As said in the anime section, School Days is unique in that it deconstructs andreconstructs the trope. The triangle can end up in massive disaster, but at other times it can either be played straight, become a Love Dodecahedron involving other people, and sometimes even evolve into a One True Threesome - it all depends on what the player aims for and the decisions they take through the story. The spoileriffic anime paragraph above only counts for The Anime of the Game, which already has a fixed plot that takes elements from what made the School Days series famous: the three possible bad endings that the player/s can get. (And we're not including the School Days manga, with totally different characterization and ending, and the spin offs and/or Alternate Universes known as Summer Days, Cross Days and Shiny Days.)
Can potentially happen in True Love Junai Monogatari, if you attract the attentions of both Mikae Morikawa and Remi Himekawa towards yourself. In fact, if you win Remi over but do not push Mikae away enough, Mikae will insult and then completely ditch you in your last date with her — something she does not do if you romance other girls, including Mikae's best friend Miyuki.
Bittersweet Candy Bowl has a number of these, generating a Love Dodecahedron. However, the comic focuses mostly on SandyxMikexLucy, though AbbeyxDaisyxAugustus also appears, as does AbbeyxDaisyxMike].
Platinum Grit has the protagonists in a massively dysfunctional love triangle. Jeremy likes Nils but beats himself up over thinking naughty thoughts about her. Nils enjoys teasing Jeremy but is terrified at the thought of developing a real emotional attachment to anyone and is retreating into no-strings casual sex with strangers. And Kate (who may or may not have had a badly thought out same-sex relationship with Nils at some point) initially just felt pity for Jeremy, but is finding herself more and more attracted to him, but she's too embarrassed to come right out and say it. If she did, he'd almost definitely beat himself up for suddenly thinking naughty thoughts about her too.
In My Roommate Is an Elf, both Jacinda and Sebastian more or less start flirting with Griswold as soon as they meet him. Griswold seems more attracted to Jacinda though, but Sebastian sees it all as a competition.
Girl Genius has several with the main one being the one between Gil, Tarvek and Agatha.
Amongst the minor characters there is a love quadrilateral. Both Violetta and Snaug are interested in Moloch who in turn fancies Sanaa, who in turn possibly fancies Tarvek, thus linking the triangle and quadrilateral into a septagon.
There was also a love quadrilateral in the previous generation, with Bill Heterodyne, Klaus Wulfenbach, and Aaronev Sturmvoraus all in love with Lucrezia Mongfish. She wound up marrying Bill, but now Gil and Tarvek, the sons of the two unchosen suitors, are courting Lucrezia's daughter, Agatha.
In Sandra and Woo Sandra and her friend Larisa have both fallen in love with Cloud who seems to prefer Sandra.
Homestuck has a few, complicated by the great amounts of Ship Tease and the fact that the trolls have several different kinds of romance, in each of which only they are expected to be monogamous.
Firstly, we have the Dave-Terezi-Karkat-Jade love square. Dave and Terezi are friends, with Dave possibly having feelings for her; Terezi and Karkat have something going on in the red quadrant but it's unclear exactly where they are; Karkat canonically gets a crush on Jade later on, who rejects him; and Jade has had a bit of Ship Tease with Dave.
Here's an update on that: Terezi has now separated with Karkat as matesprits, if they ever were in the first place, presumably due to a) Karkat seemed to want to be in every quadrant with Terezi and b) their relationship was continuously rocky and on-off, both of which frustrated her. Terezi now appears to be seeing Dave, but this is yet to be confirmed. Jade is now dating Davesprite, which John highly disapproves of (though not in a Crazy Jealous Guy-type manner).
Secondly, we have the Kanaya-Vriska-John-Rose (and Gamzee-Tavros-Jade) dodecahedron. Kanaya canonically had a crush on Vriska, who never found out; Vriska canonically has a crush on John, who returned her feelings at first but doesn't seem to anymore; John has some Ship Tease with Rose; and there is some indication Kanaya developed a crush on Rose, though it was more likely to simply be admiration. To complicate matters, Vriska also had a..."thing" for Tavros, who didn't feel the same way, instead crushing on Jade, who also rejected him, thus bridging the two squares. And not to mention when Gamzee hit on him that one time.
Among the trolls there's also Eridan and Sollux both having feelings for Feferi (and Eridan having unrequited black feelings for Sollux), and both Sollux and Equius having red-ish feelings for Aradia, who seems to maintain a simple friendship with Sollux, while not interacting with Equius.
Finally, Jane, Dirk, and Roxy all have a thing for Jake, plus sometimes Dirk's autoresponder, and Roxy also has a bit of a crush on Dirk as well, while Jake has a crush on Aranea, who doesn't really feel the same way, though admits it's not out of the question.
The Marten-Faye-Dora triangle was seemingly resolved in Questionable Content, but lingering angst doomed Dora and Marten's relationship.
Marten, Padma and Elliott had a very short-lived triangle as well.
Karin-dou 4koma: At Seren's suggestion, Ran and Shizuki decide to resolve their rivalry over Tamaryu by having Shizuki become Tamaryu's wife, Tamaryu become Ran's wife, and Ran become Shizuki's wife. Tamaryu immediately mauls Seren and runs off.
Mike/Marsha/April is a pretty basic one, in that Mike and Marsha are deeply in love and April just trying to get in the way because she's crazy (and possibly being influenced by Satan). ...okay, maybe not that basic.
Dave/Margaret/Blue is all over the map. First, Dave was just Margaret's stalker. Then Blue (Mike's sister) came to visit, fell for Dave, and started stalking him a little bit. Dave and Margaret slept together to save Dave's life (though he didn't realize it until after) because she does love him, just not romantically. Then Dave and Blue slept together to help make him feel better, and around and around it goes. Mike points out that eventually Dave is going to have to choose between the two, and it's not as easy as Betty and Veronica (the friendly and understanding Blue is the Betty and the crazy survivalist Margaret is the Veronica). If Dave chooses Margaret, he will have to deal with Satan. If he chooses Blue, he will have to deal with Blue's mother. And, in all seriousness, no one knows which is worse. In the end Dave eventually chooses Blue, after having a long talk with Margaret and realizing he's just obsessed with her.
In Red vs. Blue, Lopez-Sheila-Caboose would form a "weird, horribly disgusting love triangle" if Seila was more aware of Caboose's feelings for her. (Note: Sheila is the blue teams' sentient tank, Lopez is the red teams' robot and Caboose is an idiot).
As Told by Ginger, "April Fool's": A is Dodie, B is a boy named Dustin, and C is Macie.
As Told By Ginger again, "Love With A Proper Exchange Student" (the next ep in production order, actually): A is Dodie again, B is an exchange student named Joaquin, and C is Ginger.
Dodie seems to always be at the losing end in these love triangles.
Hey Arnold!, "Gerald vs. Jamie O": A is Gerald, B is an older girl named Chloe, and C is Gerald's older brother, Jamie O. Helga/Arnold/Lila, Helga/Arnold/Ruth, and Brainy/Helga/Arnold. This show lives off of this trope.
It was even parodied in the episode "Quantity Time" when Helga and Big Bob went to see a musical called RATS. The singers started this:
Gargoyles had this for one episode after Goliath's daughter, Angela, joined the Clan. After months of thinking that they would never see a female Gargoyle again (except for one who's psychotic, genocidal, and Goliath's ex), the young trio Gargoyles instantly leapt at the chance to woo this comely newcomer, creating an annoyingly aggressive romantic rivalry for her. Eventually, Angela angrily put her foot down on this and told them to be patient for her to respond, while consoling them with the delightful fact that she has 15 sisters back on the magic island of Avalon. (For the record, she chose Broadway; Brooklyn would find a mate while being Unstuck in Time and the creator said Lexington would eventually get a male suitor.)
There's also a subtle one between Goliath, Demona (the aforementioned ex), and Elisa. The girls don't directly fight over Goliath, but he's becoming close to Elisa at the same time that he's realizing that his relationship with Demona can't be salvaged. Demona does hate Elisa more than she does other humans, but it's unclear whether or not that's because she recognizes her as a romantic rival on some level.
More triangular examples include the one between Princess Katharine, the Magus and Tom, and the one between King Kenneth II, Finella, and Constantine.
You won't believe how many Love Triangles fit into Chip 'n Dale Rescue Rangers. There is Gadget who is admired by Chip and Dale. Then there's Tammy's mad crush on Chip, combined with Gadget making her jealous. Gadget herself shows too much interest in Sparky for both chipmunks to just sit and watch. Monty is heartbroken since Désirée hooked up with Errol. And so forth...
And just to dust off an old classic, in Popeye, Olive Oyl is always being courted by Popeye or Bluto, who compete for her affections. Olive will sometimes favor Bluto depending on the short, and she's rather fickle, prone to suddenly leaving Popeye for Bluto because she's "tired of" him. Despite this, Popeye always takes Olive back after letting Bluto have it; except in "Barnacle Bill the Sailor" where he snapped at her and put her in her place at the end, which might just be a Crowning Moment Of Awesome for Popeye.
Subverted in Avatar: The Last Airbender. The ONLY episodes where Aang isn't in love with Katara are the ones in which she is in love with or being pursued by Jet. That's two episodes.
In The Legend of Korra, there was one prior to the series between Aang's son Tenzin, Toph's daughter Lin and Pema. In the end, Tenzin married Pema, with some implication that Lin is still a bit bitter about the whole thing.
And then there's Korra/Mako/Asami. Both Korra and Asami like Mako while Mako likes both Asami and Korra.
Even more complicated if you factor in Bolin's crush on Korra which started out strong, cumulated in episode five, and due to...circumstances, seemingly faded away afterward.
Initially ends with Mako and Korra as the Official Couple, after Mako realized he was in love with Korra and Asami noticed. Their breakup seemed fairly amicable, however. Is reignited in season 2, when Korra and Mako get into a fight and break up, Mako and Asami begin to re-explore their feelings for each other, and an amnesiac Korra returns and wants to rekindle her relationship with Mako. Turns out that Korra wasn't an amnesiac after all - she was just pretending because she didn't want her relationship with Mako to be over. In the end Mako and Korra both accept that their relationship won't work out - leaving every side of this triangle single by the end.
The Daria/Tom/Jane triangle was a major arc throughout Daria's fourth season.
Young Justice has had three love triangles: Kid Flash/Miss Martian/Superboy, Miss Martian/Superboy/Artemis, and Aqualad/Tula/Garth. As of the first season finale, all three of these have been resolved. Aqualad discovers that his crush Tula and his good friend Garth are a couple and eventually pulls a I Want My Beloved to Be Happy. Miss Martian chooses Superboy, but they break up between Season 1 and 2 because of Miss Martian's abuse of Mind Rape. And Kid Flash and Artemis got together and are still going strong five years later.
In one episode of Teenage Mutant Ninja Turtles, Shredder dopes three of the Turtles with a pizza laced with a love potion (Donatello demurs, because he's driving), causing the three to fall in love with Irene, and fight with each other over her affections. She's oblivious to the fact that she has three mutant turtles lusting after her, but is delighted that she is suddenly the target of three anonymous suitors.
The entire franchise has always had one as the source of the conflict between the Hamato Clan and the Foot (primarily the Oroku Clan). The beautiful, but ultimately ill-fated Tang Shen is the object of affection for both Hamato Yoshi and another suitor (in most adaptations, Oroku Saki, though in the original comics it was Saki's brother Nagi), and always chooses Yoshi over the other suitor. Said suitor then gets jealous, and, well... you know the rest.
Sidekick has Eric who likes Vana who hates him, but Kitty likes Eric to an almost stalker level.
Kick Buttowski has Kendal Perkins who secretly has a crush on the title character, however Jackie Wackerman, better known as "Wacky Jackie", more or less has a crush on him first.