Shin's jealousy towards Kenshiro (his best friend) for marrying Yuria (the love of both their lives) resulted in a bloodbath that shattered their friendships and kicked off the post-apocalyptic tragedy that is Fist of the North Star.
The trio of Kenshiro, Mamiya, and Rei. Rei is in love with Mamiya but she nurses a soft spot to Ken.
Because they were hired around the same time, the three servants Finnian, Bardroy, and Mey-Rin of Black Butler fit this once they begin to adjust to normal life and each other at the Phantomhive Manor. Tanaka arguably doesn't change anything because he doesn't interact or do much in general compared to these three and was already working for the Phantomhives before the three were hired. Snake was hired by Ciel in the current storyline, but he prefers to be in the company of his pet snakes, thus still leaving the dynamic of the original three Phantomhive servants unchanged.
Reversed in Tekkaman Blade 2. The main team of new Tekkamen are Two Girls (Yumi and Natasha) and One Guy (David).
And... double reversed with the pilots of their support craft. They even line up on opposite-gender pairs - Goliate with Natasha, Hayato with Yumi, and Anita with David.
Incidentally, the trope is played essentially straight in Tekkaman Blade, with D-Boy, Noal and Aki, and with the subordinate Tekkamen Axe, Lance and Sword (the woman).
Tatsuya, Kazuya, and Minami in Touch, twin brothers and the girl next door who played together since they were born, until "we noticed one of us was a girl" (as Tatsuya puts it). Except only one of the boys is competing against the other, the stakes are the high school baseball championship of Japan, and Minami is a strong athlete in her own right.
And, in the show proper, Spike, Jet and Faye form the core trio of the Bebop's crew.
Tohru, Kyo, and Yuki from Fruits Basket. If you're confused by the names, Tohru is the girl.
Haji, Saya, and Solomon from Blood+ fit this one well, even though only two of them grew up together. They pack the vast majority of the romantic tension this creates into one episode, which actually seems to work out pretty well.
Momo Hinamori, Renji Abarai and Izuru Kira in Bleach, though without the romance elements. Unusually for this trope, the girl is the one pitted against the guys, when she's under the manipulations coming of her love interest and boss, Big Bad Aizen.
Isamu, Myung and Guld from Macross Plus: we learn during the flashbacks that they grew up together and were good friends... until Guld became jealous of Isamu's relationship with Myung and tried to rape her.
Chiaki, Kousuke, and Makoto from The Girl Who Leapt Through Time. Both boys show interest in Makoto, Makoto tries to understand what she feels for both boys especially when other girls start making their move, and things become even more complicated due to Makoto's time-leaping habits.
Several scenes in Gankutsuou hint that Edmond, Mercedes and Fernand had this kind of relationship before Fernand decided Edmond needed to disappear quickly.
Albert, Franz, and Eugenie have a similar relationship, though with slightly different dynamics. Since all three parties are basically good, forgiving people, it's much less destructive than the above.
Gundam 00 have four versions; An adult version (Hank Hercules, Sergei Smirnov and Sergei's wife Holly), a familial one (the Trinity siblings), The Aces of the three world powers (Graham Aker, Patrick Colasour and Soma Peries), and a more traditional one (Setsuna F. Seiei, Saji Crossroad and Saji's girlfriend Louise Halevy).
Used sorta at the beginning of Project ARMS, when it's Ryo, Hayato, and Katsumi. Except that Hayato kidnaps Katsumi before he becomes Ryo's friend and has no romantic interest in her - he just wanted her as bait and politely lets her leave when Ryo shows up. And then not too long after, Katsumi is taken out of the picture and another guy and girl (Takeshi and Kei) enter.
Mikado Ryugamine, Masaomi Kida, and Anri Sonohara in Durarara!! of course. Of course, said trio turn out to be the leaders of Dollars, leader of the Yellow Scarves, and holder of the original Saika (which also has a Hive Mind with its children) respectively, also known as the gangs who are at each others throats.
In Fullmetal Alchemist, we have Ed, Al and Winry. Granted, they're not together all the time, but anyway...
Mugen, Jin, and Fuu from Samurai Champloo. Mugen and Jin are always at each other's throats, and Fuu is pretty much at the center of the whole plot. She frequently gets kidnapped, and one of the guys always saves her. Because of this, many shippers see them as an excellent example of OT 3.
The main power trio of Ghost in the Shell consists of Tosuba, Bato and the Major(a woman). Though there is chemistry between the Major and Bato Tosuba is a married man.
Chihaya, Taichi and Arata from Chihaya Furu. Actually Arata gets shipped off to another district early in the story, but it doesn't stop Chihaya from thinking of themselves as ˇThree Amigos! forever and reminiscing Arata constantly (much to Taichi's chargin).
Moto, Midou, and Hiroki of Bokura no Kiseki qualify as this during middle school, and maintain a close friendship even during high school. Moto has an obvious crush on Hiroki, and Midou is a Shipper on Deck for the two of them. However, when Moto confesses to Hiroki, she turns him down.
Looking only at the main masters in Soul Eater we have Maka, Black☆Star and Kidd.
In the Legion of Super-Heroes, Cosmic Boy, Lightning Lad/Live Wire, and Saturn Girl often fit the trope, depending on who's writing them. Particularly evident in the post-Zero Hour reboot continuity, with Cosmic Boy as the Standardized Leader, Live Wire as The Lancer, and Saturn Girl as, naturally, The Chick.
James Howlett/ Wolverine/Logan, Dog Logan, and Rose from the comic book miniseries Origin.
In the regular X-Men canon: Cyclops, Jean Grey, and Wolverine.
In the DCU's Big Three, we have Superman, Batman, and Wonder Woman. Bats and Supes often disagree on the proper course of action during crises, especially if hostages are involved. Wonder Woman, meanwhile, has spent time in love with both of them (first Superman, recently Batman). While the rivalry between Batman and Superman has only ever boiled over when one of them is mind-controlled into evil-ness (as in Batman: Hush, when Superman is controlled by Poison Ivy and Batman gets to has to kick his ass), they rarely spend much time actively plotting against one another, and are stable enough to form the cornerstone of the Justice League of America.
The classic Doom Patrol: Robotman, Negative Man, and Elasti-Girl.
In 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.
Reversed in 28 Days Later. The surviving trio are a woman, a girl and... uh... Cillian Murphy. The initial group of survivors is more conventional- chilly Action Girl Selena, a male survivor and Cillian as Jim, but, well... let's just say it doesn't work out.
Across the Universe has a two-guys-and-a-girl layout at times, but it helps that two of them are siblings. Among certain fen, though... If it's not Max hating Jude for laying a hand on his virginal little sister, Jude pining after Max and settling for the nearest female to hand, Max harboring some very un-brotherly affections, or a really Squick-y threesome... it's got a resident Mary Sue taking up the space in the middle.
Slumdog Millionaire is made out of this trope (well, this and Flashbacks). Salim and Jamal are brothers. Otherwise, this trope is played dead straight, with Latika in the remaining role.
The Green Hornet sets up Britt Reid, Kato and Lenore Case as this instead of the Power Trio you'd expect in most hero movies. Not only do the Ho Yay and Cock Fight take place, but Lenore has been unwittingly planning the Green Hornet's every move by handing Britt her analyses of his modus operandi.
The titular Sophie Scholl, her brother Hans, and Christoph Probst from the German film, though in this case it is the girl who is the principal character.
The Australian movie BMX Bandits with a 15 year old Nicole Kidman as the girl.
Between Gary, Steven and Sam in The World's End. Steven has carried a torch for Sam since they were all in school together, and even years later resents Gary for sleeping with her. Steven and Sam end up together at the end.
The original example of this trope is probably Charles Dickens' A Tale of Two Cities.
Goethe's The Sorrows of Young Werther is another influential work based on this trope. Werther grows up with Charlotte and is a close friend of Albert. Albert marries Charlotte; Werther Wangsts himself into suicide.
Harry, Ron, and Hermione in Harry Potter. Although Harry actually has no romantic interest in Hermione, though that doesn't stop Ron from fulfilling his part of the trope.
Also works with Harry, Ron, and Ginny. In this case, Harry's afraid of My Sister Is Off Limits, missing the hints that he's the only guy Ron would be happy seeing Ginny with.
James, Lily, and Snape play the trope a bit more straight, although the two men were never friends.
The novel Soon I Will Be Invincible by Austin Grossman uses a slight veriation: evil villain Dr Impossible had a crush on Erica Lowenstein, before accidentally creating the superhero Core Fire whom she fell in love with. Dr Impossible's frequent kidnappings of Erica in his early career result from this (although "they grew apart" later and he stopped). In the end Erica resolves the plot of the novel, defeating both Dr Impossible and Core Fire.
Lampshaded in The Belgariad, as the-guy-who-is-not-the-hero assumes this will happen the moment the hero returns to his village. The hero, for his part, is not even remotely interested.
He was interested when he left, and the girl is still kind of (actually, probably even more) interested than she was at the time. But in the meantime, the hero has seen what's available in the outside world, and "prettiest girl of a certain age in a small village" is not necessarily the same thing as "actually pretty." On top of this, she's terribly naive and kind of shallow, so while the "rival" is worried the hero is going to take her away, but the hero is mainly worried he won't be able to give her away.
Danny, Joe, and Irene from the Danny Dunn novels. No romance however, as the books were written a few decades ago for kids. There has to be fanfic where someone gets Irene, though. She's too awesome.
The 1967 book "The Owl Service" is all about this trope, as three present day teens find themselves re-enacting an old Welsh legend.
Older Than Print: The central relationship of the Icelandic sagaLaxdaela Saga is one, with foster-brothers Kjartan and Bolli eventually becoming competitors for the hand of Gudrun. It doesn't end well: in the final chapter, she eventually tells her grandson "He I loved most, I treated the worst."
Alfred Lord Tennyson's narrative poem "Enoch Arden" has the more traditionally masculine of the two guys (the eponymous Enoch Arden) marrying the girl (Annie), getting lost at sea after a shipwreck, and returning home ten years later to find that she remarried his friend and romantic rival, the more intellectual and sensitive of the two (Philip), and that his children now call him father as they do not remember their own. Annie believed him dead and Philip offered to take care of her, and she fell in love with him (though he had always loved her, while respecting Enoch). Enoch never reveals to his wife and children that he is alive, for he loves them too much to intrude on their happiness, and dies alone.
Alexander Key's Flight To The Lonesome Place has Ronnie "Blue Boy" Cleveland, Luis Black, and Anna Maria Rosalita. They qualify as a Token Trio with Ronnie as white while Luis and Anna Maria Rosalita both Puerto Rican with Luis also very dark skinned.
Chase, Cameron and Foreman from House. Chase and Foreman often butt heads on differences of opinion and a drugged up Cameron sleeps with Chase. Later in the series they become more intimate, but it's only for fun. Later later in the series they get married. Later later later in the series they get divorced. However, through all of this, Cameron never really has any romantic interest in Foreman ... fair enough, given that Foreman is an enormous jerk to her ... in one episode he steals her paper (not literally, he just writes a paper on the same case and beats her to print with it because he asked House to sign off on it instead of actually reading it, while Cameron was waiting for feedback) and deliberately tries to infect her with a disease he's picked up from a patient, despite the fact that it might be fatal, feeling that that will make her work harder to find a cure.
Done with extreme subtley in Robin Hood between Allan-a-Dale, Will Scarlett and Djaq; in which both men use their feelings for Djaq as a way to galvanise Robin into action after she's been captured, and which afterwards was barely ever dwelt on again except in body language and passing comments.
Used again, far less successfully in Season Three between Much, Allan and Kate. Failed because Much and Allan had to suddenly become BFF after spending the first two seasons all but hating each other, Much completely forget about his previous Love Interest, the actors had more chemistry with each other than either of them did with the actress, and Kate was a rather pathetic Satellite Love Interest anyway. And then she hooked up with Robin, and the entire Love Trianglewas forgotten entirely.
In the third season of Merlin a lot more emphasis is placed on Gwen joining what was previously the Heterosexual Life Partnership of Arthur and Merlin to form a Power Trio destined to rule Camelot together as King, Queen and Advisor.
Beautifully subverted in Community, when best friends Abed and Troy find themselves both pining over the same librarian. Rather than risk their friendship over rivalry, their response is to approach the girl at the same time, politely explain the situation to her, and then ask her out on a mutual date so that she can decide.
The second half of series 5 and almost all of series 6 (except for River Song episodes) of Doctor Who featured the Doctor, Amy and Rory. Naturally, this trope gets even more complicated when time travel is involved: the Doctor first met Amy when she was a kid, then went straight to meeting Amy and Rory as adults thanks to landing twelve years off his target. He doesn't find out they're engaged until after Amy has already had a few adventures with him and tried to have Glad-to-Be-Alive Sex, at which point he basically abducts Rory and starts shipping them. It works.
This happened during the 2nd Doctor, 4th Doctor, and 5th Doctor's eras as well but without the romance elements.
Also Ninth/Tenth, Rose and Mickey. With the romance elements.
The reverse variant (Two Girls and a Guy) happens in at least one Monk episode. In "Mr. Monk Goes to a Rock Concert," Monk investigates the murder with assistance from Natalie and from the murder victim's girlfriend.
On Teen Wolf, a triangle is haping up between Allison, Scott and Isaac. Scott and Allison are still in love. Isaac is very close to Scott, and while a romance with Allison has been rumoured, there's a good chance if his being romantically linked to Scott as well/instead.
In The Star-Spangled Girl by Neil Simon, Norman becomes obsessed with Sophie, who can't stand him. Norman's roommate and best friend Andy resents Sophie for her politics and the fact that Norman is letting himself fall apart over her. Sophie hates Andy just as much for his politics and for being Norman's enabler. They hate each other. Yup. Sure do. Oh, you know where this is going.
In recent years, BioWare has taken to giving you two starting party members, both human, one man, one woman. So, with a male player character...
Though the social dynamic in Dragon Age: Origins is rather different, as Alistair and Morrigan hate each other. Mass Effect 2 fits best, as that is the only one in which there is an explicit attraction between your two starting squadmates (though it's in the past, and neither has any problem with Shepard dating the other).
Also in Dragon Age II, one of the starting companions is your sibling, the other becomes a widow within minutes of meeting her, and is not a romance option.
The Kingdom Hearts franchise is built around this, with almost all the Original Generation protagonists falling into these groups. The original, franchise-long examples are Sora, Riku and Kairi, who even have Theme Naming all to themselves. Played 100% straight - Riku is the Rival Turned Evil because he wants to be the one to save Kairi, and doesn't want the rival who's always been one step behind one-upping him as The Chosen One.
Ven, Terra, and Aqua in Birth by Sleep are a parallel to the above trio, they even have the exact same Theme Naming. There's less implied romance, with Terra and Aqua taking a more "parental" role to Ven, who's only a few years younger but has no memories besides training with them.
The two groups from Twilight Town with Hayner, Olette and Pence against Seifer, Fuu and Rai.
Maxi, Kilik, and Xianghua from the Soul Series. They're companions for the most part, but in the fourth game, Maxi diverges from Kilik and Xianghua for his own story. Kilik and Xianghua, meanwhile, are Star-Crossed Lovers...
...which will no longer be the case in the upcoming fifth game; Xianghua has a daughter, and she's wearing Kilik's pendant, which probably means that he's the father. As far as their fate is concerned, it's still unknown.
Squall, Rinoa, and Seifer from Final Fantasy VIII effectively play out the trope in spite of the fact that Rinoa doesn't enter the picture until all of them are teenagers and Seifer is already on his way from The Rival to Rival Turned Evil.
Although, to be fair, there was Selphie, and Quistis, and... you know what, I think it was a case of everybody wanting everybody else.
Quistis subverts it since although she's known Squall the longest, by the time Rinoa shows up she's long given up on being his lover and instead tries to be a replacement big sister. Fanfic writers are apt to ignore this character trait and play the trope straight.
Denim, Vice, and Kachua from Tactics Ogre. In a way, Vice is two people. Depending on whether the player as Denim chooses to do an evil act or not at the end of the first chapter, Vice chooses to do the opposite and opposes Denim. Villain Vice and Hero Vice do not resemble each other in the slightest.
Chaos Legion had Sieg Warheit (Victory of Honesty), Victor Delacroix (Victory of the Cross), and Seila Riviere - but the big split happened before the game started.
Shiki's backstory fits this perfectly in Tsukihime, with a "sister" Akiha and her real brother SHIKI as the other guy and Big Bad.
Final Fantasy V messes with this trope a bit. When the team first comes together, it's two guys and a girl. But then Faris joins, and it's three guys and a girl. But then it turns out that Faris is a girl, and it's two guys and two girls. And then, after a spoileriffic event, it becomes three girls and the token guy.
Caim, Inuart, and Furiae in Drakengard. The tension is initially over the fact that Caim is much better at protecting Furiae than Inuart, her betrothed, is, but it evolves as the game progresses into a squickyLove Triangle long after Inuart becomes the Rival Turned Evil.
Marche, Mewt, and Ritz from Final Fantasy Tactics Advance (not including Doned nor Montblanc, since they don't meet each other at the same time).
Grandia II fits this trope to a T with Ryudo, Melfice and Reena. Ryudo and Melfice are brothers, Reena is Melfice's fiancée. He ends up being possessed and killing her, after which he and Ryudo become enemies.
Asbel, Sophie and Richard in Tales of Graces. When they were children, the three carved their names in a tree as a vow of friendship.
Also Malik, Kurtz and Lobelia.
In the post-Generation II main Pokémon games, no matter how you look at it there will always be the protagonist, their rival and their...Friend/psuedo-rival. One female and two males.
Stocke, Marco and Raynie from Radiant Historia could be this, except Marco seems more brotherly to Raynie than anything.
Alternately, Stocke, Rosch and Sonja. In one Side Quest, when Stocke confronts Rosch about his feelings, Sonja overhears and runs off. If Stocke goes to comfort her, it's a Bad End. If Stocke pushes Rosch to chase after her, though, the two hook up.
Koudelka starts with the title character, who then encounters the two guys, Edward and James, during her mission.
Even though it's actually three guys and a girl, for a long part of EarthBound this is the basic formation. It even goes back to Two Guys and a Girl later in the game, as Poo leaves temporarily in order to learn PK Starstorm and rejoins them later.
MOTHER 3 has a party of two guys, a girl and a dog (for the most part) so it is an example of this trope, kind of.
The first MOTHER is the only game in the series with a party of only three members, which is a party of two guys and a girl, even when Teddy replaces Lloyd.
In Prophecy Of The Circle it's a part of the backstory of Jacind, Yaren and Calterra, a first thing out of many that would over time erode the three's friendship.
'Two guys and a girl' appears to be the fundamental adventuring unit of the RPG-influenced setting of Dominic Deegan; apparently a sort of unofficial custom or cosmic thing. Greg, Dominic, and Luna did it at the very beginning, but there are three plot-significant triads whose Love Triangle problems cause big drama.
In the backstory, Donovan, Miranda, and Karnak were an adventuring party of a dual-classed bard and swordsman, an outrageously powerful female wizard, and a barbarian warrior. Miranda and Donovan were a couple. Karnak liked Miranda. Karnak chose a very bad time and method to declare for her, then thought better of it and sacrificed himself to save Miranda and the world with little actual harm done, but it appears to have been a significant factor in his damnation anyway. The Demon Lord Karnak has recurred as various types of antagonist and problem for the entire run of the comic.
During the second major storyline, 'Visions of Doom,' we are introduced to the trio of Siegfried, Jayden, and Milov; Siegfried was the third recurring character to appear in the strip, but here he got friends and a more fleshed-out character, including his first real moral victory at the end. They reappear regularly and their story eventually becomes very tragic.
And at the end of the 'Battle For Barthis' sotryline, Mookie introduced Grench, Stonewater, and Bulgak. As in the other two cases, the group is split up when the un-chosen man winds up going to hell, but it happens earlier and a little less tragically this time. Both parties reappear during 'The March Across Maltak' and 'The Court of Karnak,' although Grench becomes much less significant after the Love Triangle is resolved, not having a prophesied destiny or an exposition-heavy Redemption Arc.
Miranda escapes this post-triangle sidelining only because she's the main character's mother and one of the most powerful magic-users in the kingdom. Also she and Donovan are possibly the most prominent Beta Couple, and definitely the one with the least issues.
Averted in Transformers Generation 1 with Spike, Carly and Chip. Spike shared episodes with either of his two friends, but there notably never was an episode in which both of them appeared. The only exception of sorts is the Japanese Scramble City OVA, in which there was one screen featuring them both with their backs turned towards each other. Starting season 3, Spike was the only human to keep the regular spot. Carly got three appearances, but the plan to feature Chip in one episode got cancelled. Carly appeared more regularly in the Japanese "season 4" Transformers Headmasters, but only in the role of wife & mother.
In the Transformers Generation 1 Marvel comics, the human trio consisted of Buster, Jessie and "O". It all lasted until the seventh issue, wherein "O" was written out of the series.
Transformers Generation 1 Dion, Orion Pax and Ariel. Ariel and Orion were in a relationship at the time of a Decepticon attack that left all three "dead". Orion was rebuilt into Optimus Prime and Ariel into Elita One, the two continuing their relationship. Dion's fate has been murky since: popular fanon suggested he had been rebuilt into Ultra Magnus or otherwise Ironhide, which later canon implied to be true for Ultra Magnus. Then an interview revealed Dion was supposed to be dead (which also was Ariel's original fate - she wasn't originally conceived to be Elita One). Then another comic stated Dion had survived and went on by that identity. Then Hasbro stated Dion was dead. Either way, the old friendship seems gone.
In the IDW comics, it's Jimmy Pink, Hunter O'Nion and Verity Carlo.
Alexis, Carlos and Rad in Transformers Armada. In the comics, Rad and Alexis were heading for a relationship when it got cancelled.
This is present, to some degree, in The Spectacular Spider Man: Peter, Gwen, and Harry have apparently been best buddies since at least the seventh grade. However, this trope goes into effect when Harry and Gwen begin dating, despite Gwen's greater interest in Peter, who at the time was dating Liz Allan.
Willy, Quincy, and Alyssa from My Dad The Rock Star. While Quincy did not have any feelings for Alyssa, his fears of becoming an unwanted third wheel when Alyssa and Willy hooked-up lead him to attempting to sabotage their relationship. The three all work it out within the span of the episode, though.
The furlings from Once Upon a Forest comprise of this. Interestingly romance never gets between the trio, but that may be because they're children and they're separate species. Abigail, the girl of the group, develops feelings for a field mouse who isn't part of the trio.
Ben, Gwen and Kevin in Ben 10: Alien Force; the romantic element is missing from part of the triangle, in that Ben and Gwen are first cousins and grew up almost like brother and sister. Still, Ben and Kevin have something of a rivalry, since Kevin was formerly an enemy of theirs, and Kevin and Gwen's mutual attraction factors into the boys' antagonism.
It is usually played straight in fanfiction, however.
In the Teenage Mutant Ninja Turtles episode "The Tale of Master Yoshi", we discover that Splinter's owner/father figure, Hamato Yoshi, grew up forming part of this configuration, with the other guy being Yukio Mashimi (his best friend/adoptive brother) and with the girl being Tang Shen, (their adoptive sister/mutual love interest). While things were initially fine, everything goes south once Tang Shen chooses Yoshi, resulting in a quick and tragic end to the friendship.