The first assumption in a relationship described as a Love Triangle is that someone has got to go. There is a lot of good drama/comedy in that little conundrum.
Even if the writer chooses to challenge that assumption, there will be some pretty tricky footwork required to make it all come out to a resolution.
Here are the possible triangles. Put one to work.
Note: These are all point-in-time snapshots. Could be the situation at the beginning or it could be after some story has gone by. These are often used to construct different facets of all sorts of Love Polyhedra.
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1. Alice wants both Bob and Charlie, but cannot decide which. Both are unrequited or their wishes are unimportant.
The 1994 movie Threesome uses this setup with two men and a woman.
She's The Man with the crossover setup. Viola becomes Sebastian, and falls in love with Duke. Duke then falls in love with Olivia, and Olivia falls in love with Sebastian (really Viola). There's a bit more to it than that - the real Sebastian, Viola's brother, is dating Monique so she hates Olivia, but Olivia goes out with Duke to make Viola jealous. Coupled with the reason why Viola crossdresses in the first place, it's also a Love Dodecahedron.
3. Bob and Charlie want Alice. Alice is neutral toward, or not initially thinking in romantic terms of Bob and Charlie, or thinking about them at all. Not a bad situation to be in, if you are Alice, though obviously a bit less so for Bob and Charlie. Some Alices will be tempted to see if there is any Type 7 (below) potential in the situation.
Anime and Manga
The manga Ichiroh! gives us 'a' as main character Nanako with 'b' as her childhood friend Shino and 'c' as her brother. Nanako is painfully aware of their feelings, but can't seem to discourage either of them, leaving the two to fight between each other for her.
Ren (A) and his two best friends (a guy and a girl) in Kaze no Stigma, who are quite jealous of each other.
Both versions of Mai-HiME contain type 3 triangles involving the main character(s):
(anime) Reito (B) and Yuuichi (C) are both attracted to Mai (A).
(manga) Yuuichi (A) is sought after by Mai (B) and Natsuki (C), since he is the "Key" to both of their HiME powers. Eventually, Shiho also tries to claim him.
In Axis Powers Hetalia, Greece and Turkey both like Japan who backs out of telling them which one he prefers. Word Of God even defines their relationships as Greece —> Japan <— Turkey. However, considering that the ratio of Ship Tease is heavily skewed towards Greece and Japan, many fans see this as more of a Type 4 triangle with Turkey at A, Japan at B, and Greece at C.
In Naruto in Part I, the two tsunderes Sakura and Ino (B and C) both like Sasuke (A), who shows no romantic interest in either. Both Naruto and Lee (B and C) have feelings for Sakura, who is in love with Sasuke and thus strongly rejects their initial advances.
In Digimon, Davis and TK are in love with Kari, or at least Davis is. Many fans support either, and the pairings have been known as dakari (for Davis) and takari (for TK). It is unclear who her husband is at the end of Season 2, or if it even either of the two.
In Mai-Otome, Arika(B) and Nina(C) over Sergay (A).
A developing case in Attack on Titan, with both Ymir and Reiner having feelings for Krista. The former is officially stated to be in love with her, while the later has a crush that may or may not be serious. Krista, for her part, hasn't shown that she considers either one of them as anything more than a friend.
See Sabu: Osue is at 'a', in the sights of both Sabu and Eiji, two friends from childhood.
Paint Your Wagon starts off as this, with Ben winning Elizabeth at auction but with the Stranger longing for her as well. Rectified temporarily when Elizabeth proposes turning this into a Type 7.
Star Wars appears to set up this scenario with both Han and Luke being potentially interested in Leia, but later it becomes apparent that it's Han she has feelings for and Luke is her long-lost brother.
Anne (A), in Anne of Avonlea, with Chalie Sloane (B) and Gilbert Bylthe(C) — although it is hinted that she was in denial of a type 4 towards Gilbert all along, or even a type 7 with Roy and Gilbert in Anne of the Island.
Peanuts starts here. Peppermint Patty and Marcie are both shown interested in Charlie Brown. Chuck/Charles isn't quite noticing of them...
Most noticeable of late is the situation in which Daniel Bryan found himself with Monday Night RAW's resident twin tag-team teases/terrors, the Bella Twins (until they found him making out with Gail Kim — they didn't take that well).
Agatha (A), Gil (B), and Tarvek (C). Bit of a cross with Type 1, as Agatha likes both of them, and oscillates between which one she wants, but has not committed either way. An added twist is that Gil and Tarvek are each aware of the other, and hate each other (for reasons unrelated to Agatha, and have yet to be made clear).
Agatha and Gil have started to express their feelings for each other, making this somewhat type four-ish (With Tarvek as A and Agatha at B).
Trudy and Ki of General Protection Fault are both attracted to Nick, who is struggling with his feelings for the two of them and his own attitudes toward relationships. After Nick and Ki get together, this turns into a Type 4.
In Homestuck, both WV and AR are infatuated with PM, but PM either is oblivious/doesn't have a preference.
4. Alice wants Bob, who is already in a relationship with Claire. Bob is unaware/neutral (leaving Alice as a Hopeless Suitor), or is aware of the interest and can't/won't reciprocate. As the most complicated set of relations which is compatible with both monogamy and heterosexuality, this is very common.
Yuzuru Narazaki (A), Yukari Fujimaru (B), and Ryo Katsuragi (C) in Penguin Revolution, first professionally (Yukari is Ryo's manager, and Yuzuru stages a very determined campaign to convince her to become his manager instead) and then romantically, with the added twist that as far as Yuzuru knows, Yukari is a man.
Strawberry Panic! centers on two of these triangles. The first consists of Tamao, Nagisa, and Shizuma as A, B, and C respectively; the second is Yaya, Hikari, and Amane.
In Ranma One Half, (A) Ryoga Hibiki wants (B) Akane Tendo, who is, more or less, engaged to (C) Ranma Saotome. Akane doesn't seem to notice Ryoga's crush on her and remains romantically interested in her fiancée. In fact since so many people are interested in either Ranma or Akane, "a" is where most of the characters in the Love Dodecahedron are at baseline, with some additional complications thrown in due to the existence of characters e, f or g.
Future GPX Cyber Formula has Randoll (A), Asuka (B), and Hayato (C). Randoll wants Asuka, but she doesn't return his affections and instead shows her affections to Hayato towards the end of the TV series. In fact, Randoll promised Asuka that if he wins the Spanish race, he gets her kiss. Needless to say, it doesn't work.
In Kimagure Orange Road, Hikaru Hiyama (A) wants her "darling," Kyosuke Kasuga (B), who thinks of her as a younger friend or a complete annoyance (depending on the situation). Kyosuke, in turn, is very interested in the cool and standoff-ish Madoka Ayukawa (C), the first girl he met when he moved to his current neighborhood. On the surface, this is appears to be a type 5 case, except that Madoka feels the same about Kyosuke, she just doesn't always show it.
Ai Yori Aoshi, with Tina, Kaoru, and Aoi at a, b, and c, respectively. Complicated by the fact that Tina and Aoi like and respect each other.
In the Mai-HiME anime, Takeda is A, with unrequited feelings toward Natsuki at B, who is (implied to eventually return Shizuru's feelings). In the ending, despite Mai and Yuuichi becoming a couple, Reito and Shiho are still persisting in their attempts to get together wtih the partner they desire.
In Nogizaka Haruka no Himitsu, Shiina is A to Yuuto's B and Haruka's C. At least she gets to spend one episode largely with him in episode 7 of Season 2.
In Freezing, Rana is A to Kazuya's B, and Satellizer's C. Not that it stops Rana from trying to get him anyway.
A rather twisted variation shows up in Full Metal Panic!, with Gauron at (A), Sousuke at (B), and Kaname at (C).
A less disturbing one by the end of The Second Raid: Sousuke appears to have finally embraced his feelings for Kaname, while patching things up with, but not necessarily entering into a relationship with, Tessa.
A similarly twisted version occurs in Clover, with Bols at A, Kazuhiko at B, and Ora at C.
07-Ghost: Vanessa (A) loves, and is politically married to King Krom (B). Krom's true love is Millea (C), whom he made a concubine. Teito is Millea's mother.
Later on in the manga we find out Millea is at the center of a love triangle as well: King Wolf Barsburg (A) has long been in love with Millea (B) who, while still living in Barsburg, fell in love with Krom (c), resulting in the love triangle above.
The most likely theories concerning Ninjette (A), Thugboy (B), and Empowered (C) is that they make up type 4 or type 6. The lech decorating Emp's coffee table apparently wants it to eventually end up as type 8, but no luck so far.
Quantum and Woody has this with Eric/Quantum (A) carrying a torch for Amy Fishbein (B), who's got the hots for Woody (C). Woody is indifferent to Amy, but knows that Eric's infatuation with her has messed him up a bit.
Woody: "The only reason I'm even wearing this stupid costume is because you couldn't score with Amy Fishbein!"
In Naruto Veangance Revelaitons, Naruto (A) still loves Sakura (B) and resents Ronan (C) for stealing her away. Sakura is quick to point out how much better Ronan is than Naruto, but is willing to have sex with him on a whim.
Brand Upon the Brain gets bonus points for diagramming this onscreen. Guy (a) and Sis (c) fall in love with Chance (b), and also with Wendy, (b in another triangle that has the same a and c.) The individual they think is Chance is actually Wendy in drag, so both triangles resolve in Sis's favor. (Guy faints when he realizes precisely what's going on.)
The controversial film The Night Porter by Liliana Cavani. Ex-Nazi officer Max (B) has a very visceral attraction with ex-inmate Lucia (C) (never mind her husband, who isn't in the picture). (A) comes as a homosexual ex-SS colleague of Max, whose unrequited love eventually drives him to murder.
Thai's epic poem, Khun Chang Khun Phaen, has Khun Chang (A) in love with Wanthong (B), who is already a fiancée of Khun Phaen (C), and Khun Phaen is The Casanova who practically loves every beautiful girl he comes across, including Wanthong, but due to the multiple events of kidnapping and husband switching, the relationship of the three turns into type 7. Tragically, Wanthong ends up getting beheaded for not be able to decide who she really wants to live with.
In Arrow's Fall of the Heralds of Valdemar series, this is what Dirk thinks is occurring in the Love Triangle between himself, Kris, and Talia. Of course, he and Talia are really lifebonded but are in denial about it, while Kris and Talia did in fact have an affair that is now over. Add in a healthy dose of Poor Communication Kills and Drowning My Sorrows, and it takes Kris' actual death, Talia's near-death, and something akin to divine intervention to sort things out.
In The Belgariad series, Silk had feelings for Porenn...who was a) not interested, and b) married to his uncle. He gets over it eventually.
In The Heroes of Olympus everyone thought there was a triangle with Nico as A, Annabeth as B and Percy as C. Turns out Nico is (or was) actually in love with Percy, who is in a happy relationship with Annabeth.
Smallville season one: Lana is dating Whitney, Clark is crazy over Lana.
Late season one: Chloe is in a relationship with Clark, Pete has a secret crush on her.
Late season two: Lana is having a relationship with Clark, but Chloe is very, very far from getting over Clark. This situation is more or less what the trio find themselves in half of the later seasons, although Lana and Clark has always been rocky. Lex and Lionel love messing with Chloe over this.
Season four: Lana is with Jason, Clark is still crazy over her.
Season seven: Clark and Lana are together, Chloe is still not completely over Clark.
Season eight: Chloe and Jimmy are getting married, Davis is attracted to Chloe. Moves towards type 7. Yeah, Smallville has really messy relationships.
In Glee, Dave Karofsky is interested in Kurt, who is already in a relationship with Blaine. In addition, Sebastian has feelings for Blaine, setting up another triangle with Blaine as B and Kurt as C.
Arrested Development: Michael is A, Marta is B, GOB is C. Later became type 7 (and briefly type 5) when Marta returned Michael's feelings, but not long afterwards she dumped both of them when they got in a fight over her.
Frasier, for much of its run, with Niles at A, Daphne at B, and a series of Cs.
I Carly season four finale sets up like this. Sam wants Freddie, who shares an existing, complicated double sided Just Friends relationship with Carly. Freddie is is aware of Sam's interest since she kissed him, but there is nothing to suggest he likes Sam back.
EastEnders's triangle between Syed, Christian, and Amira fits here, with Christian at A, Syed at B, and the oblivious Amira at C.
Torchwood: By series 2 and 3, Gwen is at A, Jack is at B, and Ianto is at C. While Jack is quite aware of Gwen's feelings for him and ambiguously returns them, he gently pushes her toward a normal relationship with her fiance ( and later husband) Rhys. Also, while Jack and Ianto's relationship is blatantly physical, the emotional aspects are brushed over until series 3.
Noah's Arc: Three examples of this type of triangle: The first is Trey, Alex, and Guy, with Guy pining after Trey who is in a relationship with Alex. The second is Noah, Wade, and Ricky (it is revealed in the movie that Ricky has been in love with Noah for some time, while Noah is happily with Wade). Noah, Wade, and Baby Gat have a similar dynamic.
Buffy the Vampire Slayer: In season 1, Xander is in love with Buffy, who is in a relationship with Angel. In the Season 8 comics, Buffy starts having feelings for Xander, who is dating her sister Dawn.
In Kingdom Hearts (more prominently in the first episode but possibly in the others as well) Riku (A) has feelings for Kairi (B) but she and Sora (C) are hinted to be in love with each other.
In Professor Layton and the Unwound Future, Layton (C) is revealed to have been in a loving relationship back in college with Claire (B), who died in an accident. At the time of her death, there were not one but two (A) figures in this quadrangle. One was her boss (A1), and it's implied that she was aware of his feelings but didn't return them, a fact which he reluctantly accepted; the other (A2) was a fellow student at their university, and there's no indication that she even knew he existed. A2 took her love for Layton much harder, and it became hisStart of Darkness.
In Warcraft, Illidan (A) is in love with Tyrande (B), but she loves Malfurion (C), and they officially marry after the Stormrage novel. That said, Illidan is already dead at that point.
X-Change Alternative 2 has a fantastically bizarre version of this. Yuuki (a guy) has his body accidentally swapped with Sayuri (a girl). In Sayuri's body, Yuuki realizes that he loves Sayuri, so he decides to go to confess but before he gets to confess Sayuri tells him that she has something important to tell him. And then declares that she loves Mine (a girl), thinks of having her body swapped with a guy a gift by god and is going to confess to Mine. He of course decides to support her decision. When Sayuri confesses to Mine, her response clearly declares that Mine loves Yuuki. So all in all, Type 2 became Type 4. Oh and the storyline also involves Type 2 (two of them) AND Type 5. In one storyline you'll get to see 3 types of love triangles.
It is clear in the first game that Jak (C) and Keira (B) are in a relationship, yet Daxter (A) still tries to win Keira's heart. This is no longer true starting in the second game, with the introduction of Tess.
Starting in Jak II, we also get Ashelin (A), who's shown in a few cutscenes to harbor romantic feelings towards Jak (B). Although they (presumably) kiss at the end of Jak 3, Jak decides to remain with Keira (C), in the end, while Ashelin starts a relationship with Torn.
Justin, Elliot, and Sarah in El Goonish Shive. Justin's gay, Elliot's not. Earlier in El Goonish Shive, Nanase was in Sarah's place, with Sarah being in Justin's, and Elliot thinking Sarah's attraction to him isn't serious, so he doesn't respond for fear of damaging their friendship until the issue is forced.
In Faux Pas, both Cindy and Dusk are in love (or lust) with Randy, who believes that Dusk is more predatory than loving.
Misfile had this for a short while in the story with Ash (B), Emily (A), and Missi (C). Heavily disputed that is will eventually have Emily as C and Missi as A.
Nina and Ed are most certainly into one another in Between Failures. Reggie is hot on Nina himself, but neither Nina nor Ed can stand him, because he's a narcissistic twit who seems to think taking responsibility for your own fuckups is optional.
In General Protection Fault, Ki and Trudy both are in love with Nick, but Nick eventually decides on Ki. Fooker's relationship with Sharon, having just rekindled, is put on hold when he is forced to work overseas to rebuild his cover, and while Maddie O'Hara develops feelings for him during that time, his feelings remain for Sharon.
Survival of the Fittest, Imraan Al-Hariq (A) has a crush on Jessie Anderson (B), who is in a relationship with Daniel Kensrue (C).
Red vs. Blue has Caboose, Sheila (the tank), and Lopez (the robot mechanic). Caboose (A) loves Sheila (B), but she and Lopez (C) at least like each other more. She's fairly neutral toward Caboose, though, and the two are still friends.
Ron (A) wants Kim Possible (B), who is already in a relationship with Erik (C). Ron won't make Kim aware of his feelings with Erik still around because he is a loser compared to 'perfect' Erik, and Kim is too happy with Erik to think about Ron.
5. Alice loves Bob, but Bob loves Claire. Claire doesn't have feelings for either. Alice might try persuading Bob to give up on Claire. If Alice is Axe Crazy (or otherwise extremely jealous), she might attempt to Murder the Hypotenuse. Alternatively, Alice might just want her beloved to be happy, and try to set Claire up with Bob. This is the smallest situation directly involving at least three characters where All Love Is Unrequited.
This can potentially involve more than three people, with Claire having feelings for David, David having feelings for Emily, and so forth (perhaps even looping around to Zeke loving Alice in a dodecahedral version of Type 2.)
Anime and Manga
Four parties involved in Kannazuki no Miko: Otoha adores Chikane, who wants to protect Himeko, who has eyes for Souma.
In Slam Dunk, Sakuragi is in love with Haruko, who is in love with Rukawa (who doesn't seems to be interested in anything but basketball).
In Naruto Hinata has feelings for Naruto, Naruto and Lee have feelings for Sakura, and Sakura and Ino have feelings for Sasuke, who cares more about revenge than romance. In Kakashi Gaiden, Obito is in love with Rin, but never told her, and Rin is in love with Kakashi, who states he doesn't deserve her for abandoning her. Sakura confesses to Naruto, who isn't willing to accept it, and Lee's apparently still interested in her, getting quite shocked and upset at her confession.
In Tenshi Ni Narumon the main triangle involves Noelle who loves Yuusuke, who loves Natsumi, who obsesses over her dead brother. Also, on the villain side, Muse loves Eros, who loves Silky, who... fixates on Noelle. And a bit more complicated one, Raphael loves Mikael who is interested in Noelle, who loves Yuusuke. Later subverted when it turns out Mikael was in love with Raphael all along.
In Sailor Moon R, Usagi and Mamoru are in love. Prince Diamond is blinded by his love for Usagi, which makes Emerald jealous as she is in love with Diamond. Furthermore, it is hinted Sapphire was in love with Emerald, given his suspicions about Wiseman causing her death. Although Prizma and Sapphire used to be an item.
Canaan has a pretty tragic setup involving this among the villains. Cummings (A) is in love with Liang Qi (B), but Liang Qi treats Cummings like crap and only has eyes for her boss and Onee-sama Alphard (C), who she is Yandere for in a big way. Alphard, meanwhile, couldn't care less about either of them, and harbors a bit of Foe Yay toward Canaan (the D in this Love Dodecahedron), which Liang Qi murderously despises Canaan for.
In Attack on Titan, Jean's attraction to Mikasa fuels much of the tension he has with Eren. Eren, for his part, is utterly oblivious to Mikasa's feelings for him and may in turn have feelings for Annie. Mikasa even calls him out of this, demanding to know if he has "special feelings" for Annie.
The dynamic between Túrin (C), Finduilas (B), and Gwindor (A) in The Children of Húrin is best described as a type five triangle. It also has elements of type 12: Túrin is loyal to Gwindor, and Finduilas and Gwindor were ex-lovers.
One of Heinrich Heine's most-quoted poems, No. XXXIX of the Lyrisches Intermezzo, describes a situation like this: A young man loves a young woman, she loves a different man who loves and marries someone else entirely. The young woman then marries somebody who happens to cross her path, leaving the first young man distraught:
Das ist eine alte Geschichte,
Doch bleibt sie immer neu;
Und wem sie just passieret,
Dem bricht das Herz entzwei.
(That is an old story, but it always stays new; and it breaks the heart of anyone to whom it happens right now.)
In Tales of Kolmar, Vilkas, Will, and Aral are all friends. Aral has an obvious crush on Vilkas, who is aware of it but doesn't want her, while Will pines after Aral, who has no idea. When Aral gives Vilkas her Anguished Declaration of Love it's seen as a betrayal to everyone; when Will gives Aral his declaration it's seen as sweet and romantic.
Smallville early season one: Pete wants Chloe, Chloe wants Clark, Clark wants Lana, all unrequited...for now. Pete is the only one who never gets any love.
Season four: Clark is still crazy over Lana, and Chloe is still crazy over Clark.
Willow wants Xander who wants Buffy, particularly in the early part of season one. Once Buffy decides she wants Angel, it becomes quadrilateral. Ah, high school.
The Best Years had this love triangle in season one. Dawn had feelings for Trent, but he had feelings for Samantha instead.
For most of My So-Called Life, the Brian (A) - Angela (B) - Jordan (C) love triangle was of this type. Many people think it still is; the rest saw it mutate to #4. In "Life of Brian", it briefly became a rectangle with Delia (uh, A'?), but Brian quickly destroyed that with the Idiot Ball.
Charlie, The Waitress, and Dennis in It's Always Sunny in Philadelphia. Charlie is obsessively in love with the Waitress, who despises him. In turn, the Waitress is in love with Dennis, who is openly contemptuous of her. Each member of the triangle, as well as outsiders (such as Mac), attempt to manipulate the rest of the triangle for their own benefit.
It's more than a triangle, but in Downton Abbey series three, Daisy, the assistant cook, wants Alfred, a footman, who wants Ivy, the kitchen maid, who wants Jimmy, the other footman. In the words of Mrs. Patmore, the cook, "You know, the trouble with you is that you're all in love with the wrong people!"
In Dead Gorgeous, Jonathon has a crush on Rebecca, who has eyes only for hunky jock David, who regards Rebecca only as a friend.
Quite a few Miyuki Nakajima songs. Most well-known example is "Akujo," where the singer is A, the man she's singing to B, and Mariko C.
In the Vocaloid song Daughter of Evil, Rin Kagamine (Daughter of Evil, as well as princess of the kingdom of yellow) falls in love with the prince of the blue kingdom by the sea, Kaito. However, he falls in love with a girl from a green land, Miku. Since Rin does not know which girl Kaito fell in love with... It doesn't end too well for the green land.
The J. Giles Band's "Love Stinks" describes this in the first verse.
The Taylor Swift song "Invisible" is this. The narrator likes a guy who likes another girl, and the song is basically her trying to convince him to pick herself instead of the other girl.
Peanuts: Lucy (A) is (terribly) infatuated with Schroeder (B), who's only interested in playing his piano (C). Peppermint Patty (A) likes Charlie Brown (B) but he only likes the little red-haired girl (C), who has no interest in him. Sally (A) likes Linus (B) who doesn't seem that interested in anyone. Frankly, Peanuts just runs on All Love Is Unrequited.
Recently made more complicated with the inclusion of Linkara. Linkara loves Chick, who loves Todd, who loves Lupa who doesn't give a shit and Nella wants Chick to love Critic who loves the Chick but she's the Master of the Mixed Message to him.
The situation in the Legend of Zeldacartoons, edging on Type 4. Link (B) likes Zelda (C), who knows full well what a loser he is, and whenever she decides he's being cute enough to get a kiss, circumstances intervene. Meanwhile, Spryte (A) is in lust with Link, often bashing Zelda right before trying to come on to Link...who prefers his girls a little taller than three inches. (Incidentally, Spryte is one of the three girls in the series who he actually does get kissed by.)
In The Walking Dead, Carol (a) grows feelings for married couple Rick (b) and Lori Grimes (c), constantly telling them both she is in love with them, and suggesting to make a polygamous relationship work and raise one another's children together. However, she is shot down by both Rick and Lori on this idea, and berated for not focusing on being a mother to her daughter Sophia.
Sophie's Choice — Stingo loves Sophie and Nathan dearly, even if not sexually in the case of Nathan.
In Plan B, Bruno (A) tries to seduce Pablo (B) as part of a scheme to get him to break up with Laura (C). His ultimate goal is to switch places with Pablo in this triangle as he's actually straight and Laura is his ex-girlfriend he wants to get back together with by hook or crook. Except that it turns out he's not as straight as he thinks he is and Laura is the one who ends up switching places with him as he and Pablo develop genuine feelings for each other to the point where they're forced to tell her that neither of them can be with her.
It's very easy (and entertaining) to read the Sid/Michelle/Tony triangle in Skins this way.
There has been more than one gay-themed dating show where two of the contestants chose each other over the bachelor.
99% of Girl on Girl Is Hot scenarios (the girls being B and C, obviously), usually with A fervently wishing for a Type 8.
Evoked for laughs in the Fred Godfrey song "Now I Have to Call Him Father" (the protagonist is "A"):
He used to come and court his little Mary Ann
I used to think that he was my young man
But Mother caught his eye and they got married on the sly
Now I have to call him Father
In the webcomic Boy Meets Boy, Tybalt (a) tries to seduce Harley (b) away from Mikhael (c), who used to be his boyfriend. It's implied that Tybalt still has residual feelings for Mikhael, so his fixation on Harley probably has to do with jealousy as much as with desire.
Misfiled Dreams uses Jenny in A with Ash and Emily at B and C. As Emily puts it, Jenny is attracted to Ash, Emily, and the relationship Ash and Emily have.
7. The classic "affair" set up. Alice is in a relationship with Bob, but she's also in a relationship with Charlie. Both Bob and Charlie don't know each other or suspect the other connection. Much of the drama/comedy comes from all the work needed to make sure that the space between B and C stays free from any sort of information flow. A lot of geography between the two helps, or maybe a good social divide.
7a. This set up also reflects a number of "family versus romance" situations. For example, Alice is a widow or divorcee starting a new romance against the wishes of her children. Bob (her son) and Charlie (her new man) are all too aware of each other in this case, but do not have a friendly relationship. Alternatively, Bob is an Overprotective Dad, Alice is his daughter, and Charlie is her boyfriend.
Anime and Manga
In Tenchi Muyo!, both Ayeka and Ryoko love Tenchi, and he loves them back. They also hate each other, and constantly scheme to be his sole possessor. Tenchi, for his part, successfully navigates his two relationships, keeping the two in a balance in which neither decides not being with him at all would be better than sharing him.
The risqueTouhoudoujinshi "Doll Suite" takes the usual #3 love-triangle of Marisa/Alice/Patchouli and throws it into #7 territory, with Alice being (A). Marisa, in particular, seems unhappy about having to share Alice with Patchouli, but ends up agreeing to play nice.
Averted in Sailor Moon. Mina falls in love with two men named Jack and Charlie and is trying to sneak back and forth through two dates with them at once. Why is it averted? Because Jack and Charlie are really Tiger's Eye and Hawk's Eye, competing with each other over who gets to look into Mina's dream mirror.
Full Metal Panic!!!: Sousuke at A, Tessa and Kaname at B and C, both vying for his attention (and aware of the competition), with Sousuke feeling for both.
Ran from Clover loves A as a brother, but A has decidedly un-brotherly feelings for Ran. Ran, meanwhile, is also in a relationship with Gingetsu. Could also be seen as a type 4.
While it was more a problem of attraction rather than infidelity, this was a problem faced by Peter Parker back when he was with Gwen Stacy and had some feelings (mostly shallow ones) for Mary Jane Watson. Gwen seemed to have won, though her death sadly came soon after.
Reed and Sue are married with children, but Sue and Namor have had feelings for one another for years. They've only ever acted on them in alternate universes, but they've had a lot of alternate universes together. Later writers revealed that Reed was secretly also in the same situation with his first love/college sweetheart/ex-girlfriend Alyssa Moy.
Marvel is just so fond of this, especially with the X-men: X-men Scott and Jean with Wolverine James; and later X-men Jean and Scott with Emma; and now apparently Scott and Emma with Namor!
A pretty common place for superheroes in general to be. Writers are often divided on whether or not characters should end up married or what have you, so they introduce multiple love interests to muddy the waters. See Superman (Clark and Lois or Lana), Nightwing (Dick and Starfire or Oracle), or Batman (Bruce and Catwoman or Talia. Or Zatanna, Wonder Woman, Sasha Bordeaux...)
Casablanca falls into this category, with A=Ilse, B=Rick, C=Laszlo. To be fair, it happened because Ilse thought Laszlo was dead, and the ending resolves it with Rick bowing out.
Clint Eastwood, Lee Marvin, and Jean Seberg in Paint Your Wagon. Not necessarily an affair, so much as both Clint and Sam are consensually married to the female lead. At least until Marvin decides to leave town in search of gold.
In the movie Carny, a member of the road crew of a circus or amusement park (which is what the term Carny refers to) strikes up a romance with one of the customers. She decides she also likes his brother, and has sex with him, too. Apparently this doesn't bother or affect any of them, it's as if they have no jealousy at all, and have simply decided that both of them are going to share her with each other.
Men with Brooms starts off as this, with A=Chris, B=Julie, and C=Amy. Eventually, Julie and Amy sit down to hash out their differences, and it is clarified that there is really no love triangle to speak of, due to Julie pretty effectively hating Chris ever since he left her at the altar years previously.
"The Rangers Three" in The Shadow of the Avatar trilogy go somewhere here. Sharantyr just does not want to abandon either and focus her attention on only one; Belkram and Itharr know this, but stay as the Vitriolic Best Buds they were.
In The Sky Is Everywhere, Lennie is in love with Joe, but also has "physical relations" with her dead sister's boyfriend (and fiancé) Toby. She picks Joe.
Cry it out loud for The Great Gatsby, Daisy is married to Tom Buchanan, Jay Gatsby is Daisy's ex-boyfriend who's willing to do anything to get Daisy to love him again. And Daisy actually falls in love with him again, even though she still loves Tom. It doesn't end well.
Shows up in the hints of the last book in the Night World series. Soul mates exist, and are usually A<—>B-type relationships. Then a main character sees the silver thread associated with soulmate bonds going from her to two guys...
An interesting take on this appears in Randall and Hopkirk (Deceased) wherein B (Marty) is dead. Normally this would leave A (Jeannie) and C (Jeff) to get on with things, but B has come back as a ghost only C can see, constantly haranguing him about it, leaving poor A oblivious to this.
There was an episode of Bones where Brennan decided that more than one relationship was only logical, as one person couldn't fulfill all of her needs. She wound up dating a good-looking guy who pleased her sexually but bored her intellectually, and a smart but unattractive guy she loved talking with. She insisted to Booth that they were both rational and mature, and would understand... if she ever told either of them. In the end, they found out and both of them dumped her.
This is all over the place in Mad Men, where workplace affairs are as common as sunshine in June. Some of the more notable extramarital liaisons, though, were Roger/Joan (who have both moved around the triangle since they've each been married, Roger more than once), Peggy/Pete (with Pete married to Trudy), and Don/everyone.
Downton Abbey has Lord Grantham's affair with Jane, which is able to go undetected largely because of the upstairs/downstairs divide (and due to Cora being sick with Spanish flu for much of it) and because it never went far beyond kissing. The show also has 7a with Sybil (A), who is torn between her love for the chauffeur, Branson (B), and having to abandon her family and former way of living (C) if she marries him. But by the time she chooses to marry him, she's happy to leave her old life.
Skins has a form of 7a with Emily (A), her girlfriend Naomi (B) and Emily's twin sister Katie (C). Katie is initially threatened by Naomi and in denial of Emily's homosexuality, because she's afraid of no longer being the number one girl in her sister's life.
Goodnight Sweetheart is classed perfectly as this, with Gary at (A), wife Yvonne at (B), and girlfriend/fiance/wife Phoebe at (C). Luckily for Gary (who's not a good organiser), the chances of Yvonne and Phoebe figuring out what's really going on are nil, due to Yvonne being in 1993 and Phoebe in 1940. Unfortunately both of them still believe he's leading a double life with someone else behind their backs...
On Greys Anatomy, Callie, Mark and Arizona are this to a degree: Callie and Mark are best friends with occasional benefits until Callie starts dating Arizona. When they break up, she has an impulsive hookup with Mark and falls pregnant with his child. Arizona is initially quite upset, but eventually she reunites with Callie and they marry, all serving as parents to the baby Sofia. They become a type 8 when Mark (who is dating Julia but really loves Lexie, who is dating Jackson) and Arizona eventually grow to care for each other (platonically), but Mark then dies in a plane crash at the end of season eight.
Supernatural: Sam starts dating Amelia, but when he meets her father, he doesn't approve of the relationship because the two are still grieving after losing someone and he thinks they're moving too fast. He can also tell Sam is hiding something. Then Amelia finds out her husband is still alive.
Thinking about it, Overprotective Dads are often stuck in this; Dad B, daughter A, suitor C. B-A doesn't have to be father-daughter, of course; any other intense but familial relationship in which one party wants to strike out into their own romantic interest (see the Skins example above) often plays out as this. If A and B are male Heterosexual Life-Partners, and it's a more comedically inclined show, expect B to say "We never talk any more!" to A in a really whiny voice at some point.
Most "sailor with a girl in every port" situations are this (often with D, E, F, G,...)
7a is a common trope in storylines with a Gay Aesop. In older settings or works, it was often a straight-up affair situation, where a gay or bisexual person would be married to an opposite-sex Beard (C) while secretly messing around with someone of the same gender (B). In newer works, the gay or bi character may have the option of being in an open relationship with a same-gender love interest (B), but may still consider the choice of remaining closeted (C) in order to fit in better.
Half-Life 2: Eli Vance, his daughter Alyx Vance and his co-worker Judith Mossman are a mild example of type 7a. Eli and Alyx love each other, Eli and Judith may be more than friends, and Alyx does not get on with Judith.
In Shin Super Robot Wars, this trope happens with Amuro Ray (A), Aya Kobayashi (B) and Chen Agi (C). In a Space Route scenario, Amuro and Aya start complimenting each other on how swift they are in combat, Aya clearly interested in him as more than a fellow pilot. Chen continues glaring at the two of them from her place in hiding. In another Space Route scenario, Chen is pleasantly surprised when Amuro invites her to accompany him to the city.
A variation occurs in Survival of the Fittest. Brendan Wallace (A) is in a relationship with Erik Laurin (B). However, Erik is secretly in love with Bobby Barron (C), who is also in love with Erik, but they don't know about it, since Bobby is in the closet and pretending to be straight, and, for all Erik is concerned, is "off-limits".
In the webcomic The Dreamer, there is an interesting spin on it. Beatrice is seeing two men — only one lives in the 21st century and one is in her "head". It Makes Sense in Context, believe me.
In "Family," Gemma is in a longstanding relationship with Ben and Stuart. The guys each are in love with Gemma, and she reciprocates to them both. There is neither attraction nor jealousy between Ben and Stuart as they're both straight and truly devoted to Gemma. As Ben explains to his parents, "This is the love of my life, and I want to grow old with her. And this is the other love of her life, and I wouldn't dream of depriving her." (Their relationship is open, and all three engage in relationships of varying lengths and depths with others duing the series, but none of them is ever more important to them than their core family, or allowed to threaten it.)
8. Straight-up threesome. In polyamorous circles, this is called a triad. It may be a Ménage à trois. Not necessarily sexual in nature. See OT 3 and Polyamory.
Anime and Manga
Hikaru, Lantis, and Eagle in Magic Knight Rayearth end up as this — though Type 9 seems to apply, as well.
Rentarou and twins Sara and Sojyu enjoy a happy relationship together in Futakoi Alternative. Fighting is not uncommon, but there's no competition for affection or attention.
Soul Eater has Death the Kid and the Thompson sisters, Liz and Patti.
Yuki, Natsu and Kaho in SEX, though despite the existence of romantic feelings it seems to remain fairly platonic/innocent for the time being. (Ironically, considering the title of the manga, nothing sexual happens, aside of flirting.)
Rito, Lala and Haruna from To Love-Ru become this at the very end of original series.
Bedlam's Bard series by Mercedes Lackey: Eric, Beth, and Kory, the main characters of the first two books are in one of these. Definitely sexual in this case; Kory is bi, Beth is attracted to both guys, and Eric spends the first book struggling with the idea that he is bi (although he doesn't struggle very hard). Results in a happy, magical (literally) threesome, at least for a while.
Seen in Paul Kidd's Talking Animal novel The Fangs Of Kaath when one of the three main protagonists, Raschid, marries BOTH of the others, Sandhri and Yariim, who have also been feeling a growing attraction towards each other.
Jun'ichiro Tanizaki's novel Manji (or Quicksand), which has at least three film adaptations and furthermore inspired the film The Berlin Affair by Liliana Cavani. Depraved Bisexual misfit beauty Mitsuko (A) hooks up bored housewife Sonoko Kakiuchi (B). Sonoko's hubby (C) wants a part of the action, and the couple grow morbidly jealous of each other. Mitsuko drugs the two so that they can only have sex with her but not with each other (turning the situation into a twisted Type 7). The affair ends with a Suicide Pact wherein only Sonoko, betrayed by the other two, lives to tell the tale.
A Home at the End of the World (also a film): Jonathan loves Bobby and Bobby clearly reciprocates to some degree, but is also in a sexual relationship with Clare, who loves Jonathan platonically, as he does her. For a time they all live together and raise Clare's baby, who is presumably Bobby's.
Lois McMaster Bujold's The Sharing Knife: Utau and Razi appear to be a gay couple when the unworldly protagonist first encounters them; then she finds out that as well as having a sexual relationship themselves, they're married to the same woman. In their culture, birth rates are low and marriages are often dissolved if no child is conceived, to give at least one partner a chance to have children with someone else. When Utau and Sarri didn't conceive, instead of splitting up they brought a third party into the marriage (and had a little girl). It's clear in later books that polyamory of this kind is unusual and not everyone approves.
Shakespeare's Sonnets. The first 126 sonnets are full of Ho Yay and written to the Fair Youth. The following 28 are written to the Dark Lady. Sonnet 144 brings the triangle full circle by implying that the Fair Youth and Dark Lady are also lovers. Of course, there is jealousy and angst, at least on the narrator's side of things, and it's not clear whether he is still involved with either at that point, indicating a potential shift to Type 4.
Les Misérables: Implied by Hugo. Joly and Bossuet "shared everything, even, to some extent, Musichetta [Joly's unseen mistress]." Fanon says that they are in a polyamorous relationship, although it could be that Musichetta is only involved with them separately.
In an episode of Dark Angel there's a (throwaway) mention of a man who hired a detective agency on his wife only to learn that she has been cheating on him with his mistress. (What's left unresolved is whether he should consider it good or bad news.)
In Hollyoaks, Nancy and Ravi were dating, but both also desired Kris, who desired both. And both ended up having an affair with Kris, without the other knowing, though it continued (but not for long) after they all found out. An example where this diagram is not a straight-up threesome.
Skins: Those who don't see the Freddie/Effy/Cook love triangle as a type 11 see it as this. Effy is attracted to Cook and Freddie, who are both into her; however, Cook and Freddie are also best friends, to the point where their affection for each other is borderline-homoerotic. The story isn't just about Effy choosing between Cook and Freddie, but also how their mutual attraction to Effy and fights over her are changing Cook and Freddie's friendship.
Somehow strange example happens in Wicked. At the beginning Glinda and Fiyero are in a relationship, while Elphaba has unrequited feelings for the latter, but also share a lot of Ho Yay moments with Glinda. With a time, Fiyero falls for Elphaba, but still dates Glinda. It’s safe to say that at some point all three parties all in love with each other. The ones that end up together are Elphaba and Fiyero, with Glinda left behind to rule Oz.
Kim Stanley Robinson's Red Mars Trilogy: Marina, Ursula, and Vlad seem to fall into this category, though we never see them closely enough to be entirely certain.
Kingdom Hearts: Sora, Riku, and Kairi's relationship can be interpreted as a non-sexual, mix-of-love-and-friendship version of this type. The same can be said of Roxas, Axel and Xion in Kingdom Hearts : 358/2 Days.
Skies of Arcadia: It's easy to interpret the relationship between Vyse, Aika and Fina as this.
Fate/stay night series has two possible. Shirou/Saber/Rin or Shirou/Sakura/Rider. Both have even their own ending in the original game. First one culminate as a real Ménage à trois in Unlimited Blade Works Good End. Second one is implied in more family-like spirit at Heaven's Feel True End. Both are also hinted at various levels in fan-disk sequel Fate/Hollow Ataraxia and many other associated works and materials.
The 7a variant is applied to a type 8 in CLANNAD. On Kyou's route, she has feelings for protagonist Tomoya, but tries to suppress them because she knows that her twin sister Ryou also has feelings for him and instead actively tries to set the two of them up. It works, for a time, but given that it's Kyou's route, it's obvious what happens in the end. Might actually be a type 11, though, if Tomoya never really loved Ryou.
Niels: The premise of the comic is exploring the three main characters' polyamorous relationship.
The "not necessarily sexual" type in the version of the Arthur/Guenevere/Lancelot triangle seen in Arthur, King of Time and Space. Arthur and Guenevere are married and in love, Lancelot and Guenevere are having an affair and in love, Arthur and Lancelot are Heterosexual Life-Partners and Arthur knows about the affair but (mostly) is happy that his friends are happy.
Girl Genius has one of these in the making. It's yet to get sexual, but you can cut the UST with a butter knife. Agatha deffinately feels something special for both Gil and Tarvek. Who return it in spades. Gil and Tarvek also have a very complex relationship between them that is somewhere between a Bromance and a Rivalry and which neither wants to do without, so won't upset even to try one-upping the other in front of Agatha too much — well, much. Although a little too rumbuntious to call a stable triang as such, although they all care, they don't entirely trust each other. Still... anybody who tries to step between those three had better hold onto their hats, because they're going to be minced for the audacity of stepping into their dance uninvited.
9. Bob and Charlie are attracted/loyal to each other but are also attracted/loyal to Alice. Bob and Charlie might be best friends or siblings, and much of the conflict comes from wanting to pursue Alice but not wanting to hurt the other.
Anime and Manga
In Macross Frontier, Sheryl and Ranka are both in love with Alto and are good friends (sometimes acting quite sisterly) with one another. The last scene in the TV series even has Ranka declaring to Sheryl that she's not giving up on him, to which Sheryl smiles and nods encouragingly.
In Touch, twins Tatsuya and Kazuya are both in love with the same girl, Minami Asakura, who loves them both. Unfortunately, this one loses a leg early on in the series. Fortunately (well, fortunately-ish), it's not the leg she's interested in romantically.
Just one of many situations in Mahou Sensei Negima!, Yue and Nodoka are best friends who are both attracted to Negi. Being bookworms, both the girls are incredibly Genre Savvy as to how its likely to end and the presence of a triangle is even Lampshaded by them at one point.
Konoka and Setsuna both initially showed interest in Negi (enough to pactio, at least), but as the series continues it's become increasingly obvious that the two will end up together. At this point, the pairing is one step short of canon. In 2017 they get married together.
Pick a Mary Sue fic (more specifically, any Relationship Sue fic). Mary Sue is at 'a', of course, and B and C are occupied by lesser mortals who have been enchanted away from their sexual identities, loyalties, duties, and all other responsibilities by the mesmerizing beauty of her violet eyes. Naturally, Mary Sue is too noble to allow them to depart the Righteous Path.
In Harry Potter, Ron and Hermione are a case of Opposites Attract. But one thing they do have in common is their shared loyalty/affection toward Harry.
Delia(A), Eric(B) and Fitz(C) in Vanishing Acts by Jodi Picoult, only Delia and Eric are engaged, which makes it also have elements of Triang Relation #4(with Fitz as as A, Delia as B and Eric as C). Fitz eventually gets Delia, though.
In Vampire Academy, Dimitri and Rose are in love, but worry that their relationship will get in the way of their mutual duty to protect Lissa.
In Shanghai Girls, sisters Pearl(C) and May(B) both want Z.G Li (A). In Pearl's eyes, she is Z.G.'s true love, but that is not the case. Z.G. and May have a relationship and May ends up getting pregnant. In the sequel Dreams of Joy, there is a partially platonic one going on, with both Pearl and her daughter Joy actually May's child with Z.G. both wanting love and affection from him.
This variant occurs in Doctor Who. The Doctor at A, Jack at B, and Martha at C. The Doctor, wary of relationships to himself and Jack, tries to keep all three relationships platonic.
In Darkest Powers, in pure Sibling Triangle fashion, Derek and Simon are brothers and best friends, but both are very attracted to Chloe. Despite this, they don't fight about it and neither wants to hurt the other. It's to the point where Derek tries to push her away and over to Simon so that his brother can be happy, but then Simon turns around and does the exact same thing upon realizing that the one Chloe likes is actually Derek. Not that Chloe is aware of this at the time, however, and remains in denial about it for a while even after Simon tells her so.
On Glee, this happens with Jake and Ryder, who go from bitter enemies to frenemies to best friends due to their shared interest in Marley. Hell, many fans feel that the two boys have better/more interesting chemistry with each other than either of them have with her.
In Final Fantasy VIII, Seifer's henchmen, Fujin and Raijin, are utterly loyal to him. As he grows more distant, they start to depend more and more on each other, eventually parting ways with their leader.
In Total Drama World Tour Courtney (B) and Gwen (C) are good friends but both want Duncan (A). Since Courtney and Duncan are in relationship and Gwen has been hiding her feelings from both this has elements of Triang Relation #4. Very recently this has become a #10 with Duncan revealing he wants Gwen, while at least for the moment still being with Courtney.
10. Alice and Bob are in a relationship, but Bob is attracted to Charlie. How sympathetically Bob is played depends on their gender. Similar to Type 4, but with a different point of view.
Anime and Manga
Maison Ikkoku: Kozue (A) is dating Godai (B) who is two wishy-washy to break up with her, while he pines over Kyoko (C)
The last major conflict of the series to be resolved is that Kyoko is B to Godai's A and her late husband Soichiro's C. In a Crowning Moment of HeartwarmingGodai decides it's Type 8.
Interestingly done in the original Spider-Man comics. For a while, Peter and Mary Jane were in a relationship, but Peter wanted to be with Gwen. At the same time, Gwen and Harry were in a relationship, but Gwen wanted to be with Peter. Interestingly, both Peter and Gwen are seen sympathetically, despite their gender. Then again, the male is Spider-Man.
In Downton Abbey, Mary and Matthew are clearly attracted to each other despite each being engaged to other people. This doesn't last; Matthew's fiancee dies of Spanish flu, and Mary breaks it off with her fiance when she decides the dirt he plans to blackmail her with isn't enough to risk a miserable marriage to him.
Fanon likes to paint the Snake/Otacon/Naomi mess in Metal Gear Solid 4 as this. Snake(A) and Otacon(B) are even raising a child together and Naomi(C) acts as a foil for their happiness right up until she dies. Naomi is usually portayed as only using Otacon's affections to further her own goals.
11. The classic affair set up with a twist. Alice is in a relationship with Bob and Charlie, but Bob becomes attracted/loyal to Charlie. Bob and Charlie may or may not know about the other's involvement with Alice. This situation is clearly the fault of Alice, who failed to control the information flow properly. Bad philanderer! No biscuit!
Anime and Manga
In Toradora!, this is the case with Ryuuji (A), Minori (B) and Taiga (C). This is a very strange twist of Matchmaker Crush. Taiga is trying to set up Ryuuji (who starts to take care of her) with Minori (who is Taiga's best and initially only friend). Taiga develops an emotional dependence on Ryuuji, who develops feelings as well. It turns out that Minori already had a crush on Ryuuji, but also had strong feelings for Taiga. Taiga remains good friends, but doesn't quite reciprocate Minori's feelings. Since the Official Couple is actually the title of the show/novels, bummer for Minori. Since everyone in this triangle is too nice to betray anyone, this draws out for a while.
Death Note gives us L (A) obsessed with his suspects, Misa (B) in love with and obsessed with Light but teases / flirts with L, and Light (C) obsessed with L and pretending to have a relationship with Misa.
Another one crops up later with Takada (A) seeing both Mikami (B) and Light (C) but Mikami is also VERY loyal to Light...
Crystal Robot’sInvader Zim fic, A ZADR Story, starts off with this love triangle. Dib is dating OC Cassie, Cassie is Zim’s sister, and Zim is attracted to Dib. Later on, Zim and Cassie switch places, until eventually, Cassie’s relationship with Dib dwindles to a close friendship, turning the love triangle into a type 7a.
Some versions of the Tristan and Isolde story portray the love triangle this way: Isolde (A) is married to Mark (C) but in love with Tristan (B), who is in love with her, but is also loyal to Mark, who is his uncle.
Water For Elephants, kind of: Jacob feels guilty about his attraction to Marlena at first because he is loyal to August, but when he realizes how dangerous August is and how he mistreats Marlena, he grows to hate him. However, he has to pretend to still be loyal to him to protect the two of them, as well as his other friends in the circus. Marlena struggles with her residual feelings for August before finally realizing that he's terrible and Jacob is far better, but must also keep up pretenses so they don't all get fired and/or killed. Phew.
Doctor Who: Rose, Jack, and the Doctor. Rose would be A, Jack would be B, and The Doctor would be C. Rose and The Doctor are heavily implied to have feelings for one another in the first season of the new Doctor Who series. When Jack shows up, its obvious that there's something between him and Rose as well. However, while Jack initially simply seems to feel competitive towards The Doctor, the feelings seem to develop into... something else. However, it's clear that things won't go beyond flirting where Jack is concerned, and the Doctor tends to flirt back at him, making this an oddly chaste and playful 8.
Nothing "odd" about the chasteness. The Doctor had previously been presented as asexual so they didn't want to upset the old fans. Plus, it's a family show with a primary audience of kids. The fans of this OT 3 normally see it as a case of a blooming relationship tragically cut short by characterdeath.
Cook/Effy/Freddie in Skins (B/C/A respectively) reaches this point at the end of series 3. Effy loves Freddie, and he loves her back, but Cook also holds affections for her, and Freddie does not want to betray Cook by going out with Effy.
Frasier had a variation on this with Frasier playing the role of (C). Being romantically interested in a woman (A) who was in a romantic relationship with another man (B), but is herself at least very open with Frasier. Her boyfriend, on the other hand, has no idea that Frasier is a romantic rival and thinks his constant presence is a result of his undying loyalty as a friend.
Downton Abbey has this with Matthew (A), Mary (B) and Lavinia (C). Matthew and Lavinia are engaged, but Matthew and Mary are still in love with each other. However, Lavinia turns out to be a sweet girl and she and Mary become close, causing Mary to feel torn between her love and her friend. (It's worth noting that Mary and Lavinia's friendship is strong enough to make them one of the more popular non-canon ships.)
Incredibly, this is the resolution of the love triangle in Fans! (at least where the original ending left things). By the time of The Revival, they've graduated to type 8, as Rumy and Aly both decided that If It's You, It's Okay.
In King of the Hill, this is the general idea of the Dale-Nancy-John Redcorn triangle. Dale and Nancy are married, Nancy has had a years long affair with John Redcorn (and has had his son, who Dale believes is his, being oblivious to the obvious clues), but Dale has generally positive feelings about John Redcorn who he believes merely treats his wife's headaches. Comes to a head when Dale assists John with his suit to gain land as reparations from the government, and John decides to end the affair since he cannot screw over someone who has treated him as a friend.
12. Alice and Charlie are in a relationship, often married, but Alice likes Bob and Bob likes/is loyal to Charlie. Depending on genders this can be a hilarious or very tragic set up.
The relationship between the main characters in Being John Malkovich. Craig (A) and Lotte (C) are married, but Craig falls for Maxine (B) who couldn't care less about him and is enamored instead with Lotte (but only when she's in Malkovich's body at first...yeah, it's that kind of movie).
The backstory to Slings and Arrows, with A being Ellen, B being Oliver, and C being Geoffrey.
The relationships between Johnny, Lisa, and Mark in The Room.
In Arthurian legend, this is how the relationship between Guinevere (A), Lancelot (B), and Arthur (C) plays out, with Lance torn between his attraction to Gwen and his loyalty to Arthur.
Hisao, Misha, and Shizune respectively, in Katawa Shoujo. What makes this situation particularly interesting is that one bad choice on Hisao's part (when the unfortunate Misha comes to ask Hisao for "comfort") can unravel all three's relationship.
13. Alice must choose between Bob and Charlie, but Bob is also attracted to Charlie. Could arise in a gay/bisexual situation, or could be non-sexual in nature. The non-sexual setup might be something like a boy (A) wants to please his dear old Mum (B) and wants to do naked things with his girl (C). Dear old Mum likes the girl, and monopolizes her attention with shopping trips and general girl-to-girl socializing stuff that greatly reduce C's naked-fun time.
In Yuru-Yuri Kyouko is A, being interested in both Childhood Friend Yui and Mirakurun-lookalike Chinatsu (although it's more lust than love). Chinatsu in turn is head over heels in love with Yui, while Yui doesn't show any such feelings to anyone and is happy with being friends with both.
Eric in That '70s Show was in the second situation for an episode. He was dating Donna and loyal to his mother, Kitty. Kitty and Donna then start going out shopping and lunching, cutting down on Eric's happy naked fun time with Donna.