In any given fandom, if there is any Shipping going on at all, there will usually be a particular couple substantially supported by both the Canon events of the series or story, and the general consensus of the fandom regarding Fanon; even if some parts of the fandom isn't thrilled about the nature of the relationship, it's generally agreed upon that the relationship does exist and cannot be easily ignored. These unbreakable couples are usually (but not always) comprised of The Protagonist and their Love Interest, though they're often seen among villain groups as well, and what makes this unique relationship can really only be decided by the individual viewer: they are called OTP: One True Pairing.
In some cases, there may be a case of love rivals competing for a single love interest, and the fandom is wholly divided over which one most deserves to get the girl or guy, with larger fandoms developing into factions devoted to one relationship over another. Or it may just be a matter of a standard Love Triangle, or a particular corner of a Love Dodecahedron where, because a romantic relationship must consist of exactly two people, one of the competing parties is eventually going to be disappointed. And sometimes, because of Fan Dumb, this can go overboard and cause massive amounts of Fan Wank.
In an incredible display of Take a Third Option, some fans adopt the concept of the OT3: the One True Threesome. It's just like One True Pairing, except there's three characters involved; most often this means the Hero, the Love Interest, and one of their best friends with whom they've had homoerotic subtext at some point. That way everybody's happy, the sex is kinkier, and no matter who's involved, there's always a little homoeroticism happening. It's like Give Peace a Chance, only instead of Peace, it's Three-Way Sex. While these groupings are most often the result of combining two other disputed couples, there are some examples of characters who simply group together in this way without the fandom splitting them into pairs; Power Trios are especially ripe for OT3ing.
See also Type 8 in Triang Relations; while other relationships modeled by that trope may include three characters, the distinction of the OT3 is that all three characters are involved and in some degree of love with each other. On rare occasions it's been proposed as a solution to Type 7, Type 9, or even Type 2 Triang Relations.
Polyamory, naturally, ties in with this (though it can go beyond three).
Not under any circumstances to be confused with the most notorious part of the Church of Happyology.
In-universe subtext goes under Threesome Subtext.
Examples
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Anime and Manga
In Star Driver, Takuto/Wako/Sugata almost seems canon, at least if the ED has anything to say about it.
There's also occasionally support for bringing Negi into the hugely popular Konoka/Setsuna couple.
Konoka/Setsuna/Asuna has also popped up on occasion.
A Lotus-Eater Machine in the manga gives us Negi/Nodoka/Yue (Nodoka's dream), Negi/Chachamaru/Evangeline (Chachamaru's dream), and Negi/Luna/Fate (Luna's dream) It helps that it's now a very real possibility....
There is a healthy fandom for Mugen/Fuu/Jin of Samurai Champloo.
Considering that two of the three are canonically lesbians and the three of them raise little Hotaru together, Haruka, Michiru and Setsuna of Sailor Moon are often shipped as a threesome.
With Rei pursuing Mamoru in the anime, alongside the leader/lancerLes Yay between her and Usagi, the three of them are also shipped together.
Every possible threesome has been tried by the fandom of Ranma ½, but the most common ones are Ranma/Ryōga/Akane and Ranma/Akane/Ukyō. Ranma being a Gender Bender increases the possibilities.
And Treize/Zechs/Une. Or Heero/Trowa/Quatre. Or my favorite: Heero/Duo/Hilde. Duo/Trowa/Quatre gets little to no love, sadly.
Gundam yuri fans seem to like Relena/Dorothy/Hilde.
G Gundam is a kind of silly example. Canon declared Rain and Domon to be the OTP, but the fans much preferred Domon with Allenby. This trope was the result.
Ino/Shikamaru/Choji, Kiba/Hinata/Shino, etc. All the teams could be seen as ready made threesomes. There are also people who avoid the pairing wars and ship Sakura/Naruto/Hinata or Ino/Shikamaru/Temari.
The Eyeshield 21 fandom has the Three Hah Brothers and (to a much lesser extent) Hiruma/Musashi/Kurita on Deimon, with occasional other threesomes popping up on the other teams, in particular Shin/Sakuraba/Takami of the Ojo White Knights.
Kazuki/Juubei/Toshiki from Get Backers (all guys, by the way—yes, even Kazuki) fit both the "grouped as three anyway" and "ship compromise" aspects.
In the Rikkai-centered Prince of Tennis circles, there's Yanagi/Yukimura/Sanada, a popular dōjinshi threesome.
Bleach: The most popular are Soifon/Yoruichi/Urahara, Rukia/Ichigo/Orihime, Renji/Rukia/Ichigo and/or Ichigo/Orihime/Ishida. Chad/Ichigo/Orihime has started to rise ever since the X-Cution arc in some corners as well.
And those are only the saner one, others dwelling in the realm of Crack Pairings. Gems such as Ichigo/Orihime/Grimmjow, Hitsugaya/Matsumoto/Hinamori or Ishida/Nemu/Mayuri (shudders) are probably not even the weirdest one out there.
Also Ichigo/Kenpachi/Yachiru, Ichigo/Ulquiorra/Grimmjow, Aizen/Ulquiorra/Grimmjow, Aizen/Ulquiorra/Ichigo, Shinji/Ichigo/Grimmjow, Hitsugaya/Gin/Aizen, and many, many more.
Pick any three members of the F/A/C/E group (France, America, Canada, England), and it's guaranteed that you'll have a threesome that's gotten its share of fanworks. Some people even just go ahead with making them an OT4.
Yuri shippers have Ukraine/Hungary/Liechtenstein, the three girls with the most familiar and friendly interaction between them.
There's also America/England/Japan, Japan/China/South Korea, Spain/South Italy/Belgium, Spain/Austria/Hungary, Finland/Sweden/Denmark, Sweden/Denmark/Norway, Japan/Taiwan/Hong Kong, Russia/America/England, Russia/Lithuania/Poland, Belarus/Russia/Ukraine, Russia/China/South Korea... y'know what? Just pick any three characters and odds are more than likely that there'll be at least one work or shipper out there for them all together.
Suzumiya Haruhi: Kyon/Haruhi/Mikuru and Haruhi/Kyon/Itsuki. It helps that Haruhi is canonically bisexual and Itsuki is...something.
Show some Yuki love — Haruhi/Kyon/Yuki! Giving up and just making it a five-way communal relationship is fine, too.
While it's debatable if it is really three people or two, Kyon/Mikuru/Adult!Mikuru gets shipped occasionally.
In Fruits Basket, Akito/Kureno/Shigure would be an amusing OT3. But Kyo/Yuki/Tohru could also be made a case for.
The true OT3 of Fruits Basket is the Mabudachi Trio - Shigure, Hatori, and Ayame. While this grouping has died down due to a loss of focus in the later volumes, this was one of the strongest threesomes in the fandom and was very obviously helped by Shigure and Ayame playing up the Ho Yay.
There's also a slight amount of Les Yay between an OT3 hinted at within the first half or so of the manga between Tohru, Arisa, and Saki. There are several official art pieces of the three, some dialogue within flashback chapters don't exactly dispell it, and they're not at all shy about all sleeping in the same bed.
There's a lot of shipping fanfics with Kyo/Yuki/Haru.
There's some degree of romantic subtext in all of the meister/weapon partnerships of Soul Eater — having compatible souls is sort of a requirement for getting it to work. Death the Kid, however, has two partners, Liz and Patty. And his relationship with either of them seems to complement his relationship with the other. OT3 shipping ensues.
There has been a suspicious rise in Noah/Kid/Gopher, as of late... and of course, Soul/Maka/Chrona (or Maka/Soul/Chrona) is amazingly popular, though not to the point of Liz/Kid/Patti. Also, although there isn't as much fanfiction and fan work done on them, Spirit/Stein/Marie is a rather popular little bit of crack in the fandom.
Tenchi Muyo!! fans who reject the Word Of God about the large harem are sometimes more willing to accept Tenchi/Ryoko/Aeka as a simple OT3. Probably because Ryoko and Aeka get way more screen time and more focus on their feelings for Tenchi than any of the others.
Canonically, there's Emperor Azusa and his wives Funaho and Misaki.
Himawari/Watanuki/Doumeki - A Power Trio enforced In-Universe because while Watanuki adores Himawari, Himawari brings bad luck to anyone around her and Doumeki is pretty much the only person who can negate that "ability".
Magical Girl Lyrical Nanoha StrikerS is nice enough to show us Nanoha, Fate, and Hayate in bed together. Granted, this was following an enormously exhausting, impromptu rescue mission, afterwhich they simply needed to crash and sleep...
Not just them, but Reinforce Zwei up on a pillow. So technically an OT4.
OT7: Hayate, the Volkenritter, Reinforce and Agito. This may be canon, but as usual, the Yagami Family dynamics aren't shown onscreen.
Some Toy Ship fans don't mind making a Toy OT3 by bringing Lutecia into the oft-teased Erio/Caro.
It doesn't take much to turn this into a Fate/Erio/Caro scene, especially since this is mentioned as "family bonding time" right after a training session where Erio inflicted severe Clothing Damage on Fate.
Considering that Erio's seen Caro and Fate naked before, it's even easier to turn that into such a situation.
Ichino Yanagida/Akari Kanzaki/Kris Kristopher from Battle Athletes Victory is one of the earliest entirely yuri bit of OT3 shipping.
Magic Knight Rayearth has Hikaru/Umi/Fuu of the Power Trio variety, and then Hikaru/Lantis/Eagle later on, with a bonus of the latter implied to be canon.
Surprisingly, Yu-Gi-Oh! has given us quite a few to work with, from all four series. Some notable ones include Yami/Anzu/Yugi from DM, Jack/Yusei/Crow from 5Ds, Kaito/Yuma/Ryoga from Zexal, and the strange love triangle of Yubel/Judai/Johan, which gained popularity in the second half of Season 3 of GX.
Hayate the Combat Butler: Hayate/Hinagiku/Ayumu FTW. It should be noted that this was Ayumu's idea, after Hinagiku decided she was ruined for marriage because Ayumu had gotten a little turned on drying off Hinagiku (who had fainted in the bath). Quietly pretend she hadn't just been joking, please.
Should be noted that that particular comment is only in the anime, the manga is a little tamer about it. Implications are still there though. And there's other opportunities for the threesome. The 'That's it' when Hinagiku admitted she'd fallen in love with Hayate could be read for such.
Or Hayate/Maria / Nagi, or Hayate/Hinagiku/Nagi. Because Hayate needs someone else to keep Nagi in line.
Except for Hayate's 'Not-a-lolicon' comments, Hayate/Nagi/Isumi could also follow that, given that Nagi and Isumi made a Childhood Marriage Promise.
Given some comments and a flashback or two from the Greece Arc, Hinagiku/Hayate/Athena could be shipped if one felt so inclined. This is made easier due to Hinagiku and Athena both liking Hayate, Athena being his ex-girlfriend (with some feelings left on both sides) and Hinagiku having a mutual crush with him that the entire cast can see, and both girls having a close friendship with some Les Yay tones to it. Still not likely to happen in Canon but pretty well set up for any fans who wanted to ship them together.
Pokémon, thanks to its infamous shipping community, has Ash/Misty/Brock, Ash/Gary/Misty, Ash/Pikachu/Misty, Ash/May/Misty, Ash/May/Brock, Ash/May/Drew, Ash/May/Dawn, Ash/Dawn/Brock...
Lucy/Brock/Holly. They all deserve to be happy.
A good portion of the fanbase basically forgets Pikachu exists altogether if he's not actively being shipped (because he gets in the way of the OMG passion). The better writers realize that any OTP involving Ash is inherently an OT3 with Pikachu to at least some degree since he's sort of a permanent fixture in Ash's life. They will play thisup forall it's worth.
Brock's OT3 is him, Jenny and Joy. He even wrote a song about it.
In Pokémon Special any region's Power Trio (or quartet in RGBY's case) will inevitably be OT3'd but special mention goes to the DPP crew (Diamond, Pearl, and Platinum) who are basically a canon OT3, being inseparable for most of their adventure and discovering just how hard it is to be separated in a powerful Tearjerker. Their story ends with all three of them basically being parents for two baby Pokemon.
D. Gray-Man fans and their Allen/Kanda/Lavi, their Lenalee/Allen/Lavi, their Kanda/Lenalee/Lavi, their Lavi/Allen/Kanda, or their Tyki/Allen/Lavi, or Tyki/Allen/Kanda, or their Lenalee/Allen/Miranda...
Heck, there's even a very popular OT4 pairing of Kanda/Lavi/Allen/Lenalee... which is actually kind of supported in the Kuro no Bansankai Drama CD. It pretty much consists of Allen, Kanda, and Lavi being tricked into getting absolutely drunk and answering questions about their true feelings about Lenalee (with them whining about what troubles them about her, and what they like about her), which ends with them summarizing that they all like her. And with the way that they all seemed very amiable and non-possessive about it, a foursome doesn't seem entirely out of the question...
Schneider/Wakabayashi/Kaltz, Hyuga/Tsubasa/Sanae and Misaki/Tsubasa/Sanae are shipped once in a while in the Captain Tsubasa fandom.
In Love Hina Keitarou/Naru/Mutsumi seems so obvious it hurts. Besides Kei and Naru being the Official Couple of that series, Mutsumi was a childhood friend who participated in the infamous promise to get into Tokyo U and in the manga states flat out that she loves (not likes) both characters and wants to see their dream succeed.
It goes beyond that, what with Mutsumi kissing both of them, there are also plenty of hints that OT3 is Mutsumi's preferred outcome. It's also pretty clear that Keitaro made a Childhood Marriage Promise to both of them (and that they both knew about the "other woman" and were fine with it- although at their age they clearly didn't understand why it was not okay).
And in chapter 59, Mutsumi flat-out states that she made a promise with Naru Narusegawa. Which means that each member of the trio made a promise with both of the other two members. And just to make it weird, Mutsumi made Naru promise to get into Tokyo U because she knew Naru was too young to remember her first promise with Keitaro, a year before that.
Even with all that has been pointed above, Naru/Keitarou/Motoko is very popular among the OT 3 shippers.
Not with Motoko herself, though, considering the Slash Fic Shinobu calls her out with in the epilogue of the manga....
Su has openly expressed interest in becoming a threesome with Naru and Keitaro. Given her character, it's very hard to tell if she was serious or not.
Mai-HiME fandom tends to gravitate heavily toward Shizuru/Natsuki/Nao, but there are also a good deal of supporters of the main Power Trio of Natsuki/Mai/Mikoto. Those who are less fixated on the Girls Love and Die for Our Ship have been known to upgrade Mai/Mikoto/Yuuichi into a full-fledged threesome as well.
The manga seemed to suggest Natsuki/Mai/Yuichi, with Mikoto and Shizuru both Out of Focus.
X1999 fandom has this in Fuuma/Kamui/Kotori, what with Kamui constantly reiterating that Fuuma and Kotori are/were his most important people and Subaru and Sorata outright stating Kamui loved them both equally, the three characters' obvious devotion to each-other (on Fuuma's part at least before his supernaturally induced Face Heel Turn and Fuuma and Kotori's obvious romantic/sexual feelings for Kamui. There's the small issue of Fuuma and Kotori being full-blood siblings... not that this has ever stops anybody. (And some try to bypass it via making Kamui the vortex and not going beyond that)
There's also a small following for Kamui/Sorata/Arashi, what with Sorata being genuinely devoted to Arashi but also getting kinda flirty with Kamui early on...
To Love-Ru has Haruna/Rito/Lala, and chapter 162 shows that Lala not only doesn't mind but she actually wants this to happen.
You're behind the times. The addition of Kaguya gives us a character who approves of a Lelouch orgy, starting with C.C./Lelouch/Kallen/Kaguya, and doesn't exceed that only because she hasn't met Shirley or the bicurious Milly.
In the Katekyo Hitman Reborn! fandom there's a rather lot of Gokudera/Tsuna/Yamamoto and a lot of Hibari/Tsuna/Mukuro, too.
Also noteworthy: Mukuro/Dino/Hibari, Byakuya/Mukuro/Tsuna, Hibari/Squalo/Yamamoto, Yamamoto/Dino/Hibari, Yamamoto/Gokudera/Hibari.
Keiichi/Mion/Shion are a favorite OT3 of the Higurashi No Naku Koro Ni fandom. Or Keiichi/Rena/Mion. Or even Keiichi/Rika/Satoko for that matter. Without Keiichi, there is Mion/Shion/X or Mion/Rena/X. On the Toy Ship front, there's Rika/Satoko/Hanyuu.
Full Metal Panic has Kaname/Sousuke/Tessa as a rather popular OT3, as well as Kaname/Sousuke/Nami. And yes, even an OT4 of Gauron/Sousuke/Xia Yu Fan/Xia Yu Lan.
To add to the list, there are fans of Kurz/Mao/Sousuke. It probably doesn't help that both Kurz and Mao have flirted with Sousuke on more than one occasion. Or that Mao has been shown to playbetween the two of them (with both Kurz and Sousuke separately blushing at her "heartfelt" words).
Mao also has quite a bit of Les Yay with Tessa; as the resident Lovable Sex Maniac, it's hard to imagine Kurz objecting to a threeway, especially with his adorable captain.
Bakemonogatari's Koyomi/Senjougahara/Kanbaru is teased a little, and actually makes sense if you think about it. Yes, Senjougahara is in the middle (oddly, she might be the only one against it).
As of episode 14, you can probably add Hanekawa to the mix, this time with Koyomi as the vortex.
Haruhi/Hitachin Twins is a quite fairly popular OT3 ship.
Twins/anybody is are fairly popular OT3 ships. It's not uncommon to find Kyouya or Tamaki in the third slot.
More common than the OT7 is an OT5, excluding Mori and Hunny. The rationale is usually Shotacon-relatedSquick that leads to not wanting to ship Hunny with anybody... and Mori would never involve himself in anything without Hunny.
The Dinosaur King anime's Power Trio of Max, Rex, and Zoe seem to be a ready-made OT3. It helps that Zoe tried to kiss Max in episode 58 and kissed Rex in episode 79.
Just calling it now, Miyoshi/Takagi/Aoki. After a series of Love Triangle-related misunderstandings, the two girls have decided to be friends with Aoki supporting the idea of Miyoshi and Takagi getting married while she's admitted to them both that she's started to fall for Takagi.
In Angel Densetsu, Ikuno offhandedly and bluntly states that, should Official Couple Kitano and Ryoko get married, she will become Kitano's mistress to make sure they all stay together.
In Rosario + Vampire, this is Yukari's plan with Moka and Tsukune, at least after Tsukune manages to befriend her.
Tomo/Kagura/Osaka from Azumanga Daioh has gained some popularity.
Rock and Revy are obviously in love, even though they don't want to admit it. Revy and Eda are extremely close friends, and probably the only person that the other truely trusts. Eda has also been known to flirt with Rock. Rock/Revy/Eda was inevitable.
Recently Revy reacted violently to someone trying to get Rock to participate in a threesome, so that may be a little bit of Ship Sinking as far as the OT3 goes. (To be fair, said threesome proposal didn't extend to Revy herself.)
There's also Rotton/Sawyer/Shenhua, who canonly cohabitate and spend all their time together.
The High School AU manga of Tengen Toppa Gurren Lagann features Simon/Nia/Dark Nia in its ending (Dark Nia in this version is a secondary persona/imaginary friend that Nia created from being denied any friends by her overprotective father).
Rare in Death Note fic, mainly because most conceivable combinations would involve a female somewhere (and Het is VERY MUCH Ew in this fandom). Nonetheless, among the less het-averse sections of the fandom can be found Mello-Near-Halle, Light-L-Misa, Light-Misa-Takada, Light-Mikami-Misa and Light-Mikami-Takada advocates (and some Misa-Sayu-Matsuda, considering older!Sayu's fangirling of Misa as well as Matsuda's crush on her) while some rare Melloshippers throw Mello-Matt-Near together.
DearS has it as the canon ending for the manga. Takuya, here's your reward for choosing to Stay with the Aliens.
Durarara!!'s Power Trio Kida/Anri/Mikado is a big OT3. Especially because of each of them is some sort of "gang" leader, which also kinda makes them enemies, ergo, unintended Foe Yay.
Also: Walker/Erika/Kadota. It helps that Walker and Erika both explicitly ship it, though they seem to disagree of which of them would be the vortex.
Erika declares Shizuo/Kadota/Izaya to be her OT3 in a side story.
Celty/Shinra/Shizuo seems to have a presence in the fandom as well.
Izaya/All Of Humanity/Saika.
From the novels, we have Mairu/Aoba/Kururi.
Izaya/Mikado/Shizuo is also surprisingly very popular given the little interaction Mikado and Shizuo have.
Hellsing: Integra/Alucard/Seras is thought by some to be an excellent way of avoiding Ship-to-Ship Combat... though only in the TV series.
OTOH, in the Manga Seras has Pip Bernadotte... but considering how it went, both of them are in a symbiotic relationship after his Heroic Sacrifice for her, so turns any pairing with Seras (or Pip) in an OT 3 automatically. Unless you go AU.
Anderson/Integra/Alucard was a popular one a few years ago.
WORKING!!: Kyoko/Yachiyo/Jun makes for much less angsty fics.
Fullmetal Alchemist has spawned the niche ship Greed/Ling/Lan Fan, though this is less because Ling/Lan Fan shippers and Greed/Lan Fan shippers can't settle their disputes any other way and more because it actually works. The fact that Ling and Greed share a body also helps.
Peach Girl has had several of these pop up, the most common being Kiley/Momo/Toji. Kiley/Ryo/Sae is less common, but not as obscure as you'd think.
InuYasha has spawned several different combinations including InuYasha/Kagome/Kouga, InuYasha/Kagome/Sesshomaru, and InuYasha/Kagome/Kikyo as well as various others (the latter to resolve one of the most bizarre Love Triangles in manga history, given that two of the participants are technically the same person).
Nagakura/Harada/Todo appears quite often as well. They're refereed to as the Three Jokers and are usually seen together.
Being based on a dating game, Hakuouki sees a lot of this. Okita/Chizuru/Heisuke seems to be the most popular thanks in part to extensive official artwork of the three together.
Actually the most popular threesome is probably Okita/Chizuru/Saitou. Though... anyone up for some Okita/Saitou/Hijikata?
Dr. Kenzo Tenma, Johan Liebert and Nina Fortner in Monster, despite (or maybe because) of the Twincestangle. Or alternatively, Kenzo with Eva Heinemann and Roberto. Or Kenzo/Roberto/Heinrich Runge.
Infinite Stratos has the mildly popular Ichika/Charlotte/Laura threesome. Notable since there are a few hints that it could be possible in canon; Charlotte and Laura are good friends, and Charlotte has been pushing Laura to get closer to Ichika.
Admit it, do you really want the male lead to fall for the tsundere girl again? To make things worse, the three other girls in Ichika's harem (Houki, Cecelia, and Lingyin) are all tsundere and fight each other tooth and nail for Ichika.
Dragon Ball: Well, an OT 3 that's become a bit more popular recently is Android 17/Krillin/Android 18.
Goten/Trunks/Marron is slowly gaining followers among those who can't imagine the two men not being together but can see them with women as well. Marron is sometimes paired up in an OT 3 with Pan and Bra as well, though it doesn't seem as common as pairing her with Goten and Trunks.
Lately I've been seeing a lot of Future Trunks/Goten/Present-timeline Trunks.
The DBZ Kink meme has quite a bit of Bulma/Vegeta/Yamcha. And there seems to be a higher concentration of OT 3 or moresome fics there than in most other places, including some for the O T3s mentioned above.
Despite the incest-ysquick, a small part of the Mawaru-Penguindrum fandom is starting to get behind the idea of Shouma/Kanba/Himari. Two or three more throw Ringo in as well.
There are not many fans of the Mazinger trilogy (Mazinger Z, Great Mazinger, UFO Robo Grendizer) ship, but some of the shippers consider Kouji/Sayaka/Maria is a solution to the most important Love Triangle in the series (nobody seems considering Kouji/Sayaka/Boss or Tetsuya/Jun/Boss, nevertheless. Poor Boss).
It is lampshaded in this◊ Super Robot Wars Yonkoma comic:
Berserk gives us Griffith/Guts/Casca, although that could be considered completley canon in some ways- Casca and Guts are the Official Couple, Casca was definatley in love with Griffith earlier in the series, and Griffith has been very strongly implied to be attracted to Guts, with slightly less (but still present) subtext with Casca. The feelings are there, it's just that canon never gives them a chance to sort them all out before things go ''really'' wrong.
The fandom of Vampire Knight is mostly divided between Kaname fans, who ship Kaname and Yuuki, and Zero fans, who ship Zero and Yuuki. Then there are the ones who say this whole problem could be conceivably solved with a large enough bed...
Psyren has a canonical example in Ageha/Sakurako/ Abyss.
Chocolat/Carrot/Tira was almost canon in Sorcerer Hunters, and some fans still would have preferred that to the manga's actual ending. Then again, Chocolat did end up with Carrot's kids...
It's not hard to imagine a Ryuuji/Taiga/Kushieda relationship in Toradora, since both male/female pairs in the trio have unambiguous mutual attractions, and Taiga/Kushieda are about as close to Schoolgirl Lesbians as you can get without it actually being canonical.
Yuyushiki: Yuzuki/Yui/Yukari. It's an event horizon between canon and fanon.
Puella Magi Madoka Magica: Take your pick: Mami/Madoka/Homura, Mami/Kyouko/Sayaka, or Sayaka/Madoka/Homura.
Card Games
Harpie Lady Sisters, Queen's/King's/Jack's Knight, Dark Magician/Dark Magician Girl/Dark Magician of Chaos (feel free to substitute with other magicians), pretty much anything remotely human in Yu Gi Oh that comes in threes.
Danny Rand/Misty Knight/Luke Cage. They're very close, Danny/Misty is canon, and when Luke had a baby, he named her after Daniel. Aww.
Danny Rand/Jessica Jones/Luke Cage seems to be pretty popular as well.
Some people have all four together, Sometimes with Colleen Wing coming for a visit.
There are three potential Threesomes that revolve around the Barbara Gordon/Richard Grayson relationship. Starfire-Nightwing-Oracle covers the Love Triangle that revolves around Grayson, Oracle-Black Canary-Nightwing covers the Triangle that revolves around Gordon, and there are several possibilites that involve Nightwing and Batman that veer off into bizarre fan theories.
This was even Lampshaded in DC Nation recently when Kory broke Babs's brain by saying she'd not be averse to inviting Barbara to bed if Dick gave permission. Nationverse also had a possible future where Gar Logan (Changeling), Fauna, and Raven had a long-standing "arrangement" and Green Shield's experience in a Lotus-Eater Machine caused her to hallucinate that Dr. Midnite, Green Arrow I, and Black Canary were in a happy three-way marriage.
Tim/Bart/Kon, from the comics Teen Titans. A very popular ship, especially given... Young Justice. (Id + Ego + Superego = they belong together!)
When the world was a better place, Steph/Tim/Cass was a fan favorite. The memory lives on in old fanfiction.
Canon examples: Leetah/Cutter/Skywise, Skot/Krim/Pike, and Dewshine/Scouter/Tyleet of ElfQuest. Another case of Everyone Is Bi, but unique in that the "three-matings" in ElfQuest are openly acknowledged and accepted.
At one point, Savah, the wise village chieftess of Sorrow's End, actually suggests that Rayek, Cutter, and Leetah work out a threesome. Rayek doesn't go for it.
Daredevil/Spider-Man/MJ turns up every once in a while. On rarer occasions Tony Stark / Steve Rogers / Thor or Tony/Steve/Pepper Potts or the "Cold Irony" ship (Tony/Steve/Tony's Sentient Armor, because Iron + Tony = "Irony" and Steve was frozen). Or just really Tony/Steve/? (there have been Spider-Man, War Machine or Jan Van Dyne in the "?" place).
And of course, there's the fandom of Spidey/MJ/Black Cat or Spidey/MJ/Gwen... and sometimes MJ/Gwen/Black Cat
Weirdly enough, Ultimate Spider-Man implies that Gwen was trying to figure initiate this, since in a line, she mentions that she crushes more on MJ than on Peter.
Don't forget Steve/Bucky/Black Widow or Steve/Bucky/Hawkeye and Bucky/Falcon/Black Widow or Bucky/Steve/Namor.
Cosmic Boy/Lightning Lad/Saturn Girl from Legion of Super-Heroes, given the closeness of the trio in canon. Ultra Boy/Apparition/Timber Wolf from the post-Zero Hour Legion isn't too beyond the pale, either.
From the cartoon, we also have Superman/Brainiac 5/Kell-El.
Really weirdly, the Watchmenfandom has taken a liking to Laurie/Dan/Rorschach. It would only be possible in an AU. Fandom has happily provided the AUs. Fits the "canon couple (Dan/Laurie) + subtexty partnership (Dan/Rorschach)" type.
The wildly popular fan pairing is Katchoo-Francine-David; it comes pretty close to Canon sometimes and even author Terry Moore seems to think this will solve the problems, but Strangers in Paradise is one of the few series to have the characters decide on a True Threesome in order to help deal with their Love Dodecahedron. Francine, David, Katchoo, Casey, Freddie and even Tambi all alternate between being friends, enemies, sexual partners and various places in between, and also rotate living with each other as one or more of those options. Eventually they all (well, most) decide for Katchoo, Casey and David to form a sexual relationship in order to produce a child, comfort David in his final days, and because they all do love each other, even though Katchoo only really loves Francine, David only really loves Katchoo, and who Casey really loves is more complicated than even the reader thinks at the time (yes, it is more complicated than what you just read). Once David dies the series settles down into Katchoo-Francine and Casey-Tambi (!?!?!?!?!?), though even then David sticks with both pairs in spirit and via his child with Katchoo... and no, Freddie does not end up with anybody, he ends up alone. Deservedley.
Currently Magneto/Rogue/Gambit. Almost official from Rogue's side.
There's also considerable love for Bruce / Clark/Lois in fanfic as a way to resolve the opposing pulls of a popular Official Couple and a popular Ho Yay target, sometimes expanded to an OT4 of Bruce/Selina/Clark/Lois.
There's also the crackier Clark/Lois/Metropolis. (Which is sort of canon, as Lois once commented that she's much more jealous of the city than of the scantily-clad superheroines Supes hangs out with.)
And then there's the Big Three, Clark/Bruce/Diana. In Mark Waid's "The Kingdom" miniseries, the Kingdom Come versions of Clark and Diana have a son with Bruce as the godfather - and the grown-up kid refers to them as his three parents.
Also Catwoman/Ivy/Harley (see especially Gotham City Sirens), Catwoman/Ivy/Batgirl, Ivy/Harley/Batgirl, and Joker/Harley/Ivy (which is, at the very least, a canonical Love Triangle).
In the Young Avengers fandom, Patriot/Hawkeye/Speed turns up with some regularity.
In Spirou and Fantasio, the Ho Yay between the titular characters is rounded out by Seccotine's semi-canon relationship with Spirou and her Foe Yay with Fantasio.
Jason/Roy/Kori was part of what was the first wave of fanart for Red Hood and the Outlaws. After the title had ONLY been announced.
And after issue one, in which Starfire has sex with Roy, and Jason states he'd "been with" her. In what might be a conscious rejection of the trope (and/or a retcon), issue six reveals this to have been spending the entire night talking with her.
Fan Fic
Misfiled Dreams seems to be leaning towards Ash/Emily/Jenny Jr. It says something when even the characters themselves have taken notice of this.
The Top Gear fandom has a lot of OT3 fic. Sometimes the third party is a car.
. This is even acknowledged in one Joss Whedon-written issue of the canonical Buffy Season 8 comic book, where it is revealed that Buffy's sexual fantasy is a threesome between her, Angel and Spike. Even more kinky, she wears a nurse uniform. And they're chained to her.
Throw Dru in for super awesome PTB points.
Oz/Willow/Tara could solve plenty of problems.
Buffy/Willow/Xander had quite a number of supporters in the early seasons.
Warren / Andrew / Jonathan isn't that uncommon, you know...
Buffy/Willow/Tara has quite a few supporters too, especially when you consider that Willow & Tara moved in to the Summers' house and were already essentially honorary big sisters/foster moms to Dawn anyway.
Given Dru's happiness at the three of them being together again and Angelus' implied past relationship with both Dru and Spike, Angelus/Drusilla/Spike is the obvious conclusion. Darla is often included. Lampshaded in Angel when Angel and Spike each admitted to having tried for (and failed to get) a threesome with Darla and Drusilla, but that Buffy's new beau The Immortal had.
Over at sister series Angel, the unexpected family-like closeness among the original cast (Angel, Cordy, and Doyle) won many fans, some of whom 'shipped the whole threesome. This went double for Angel, Cordy, and Wes — after fans forgave the latter for replacing Doyle, that is. There's also Gunn/Fred/Wes, to reconcile the shipwars.
Another Joss series, Dollhouse, has a variety of polygonic shipping. A popular ones include Echo/Alpha/Paul, Paul/Echo/Mellie, and Echo/Alpha/Whiskey.
In Due South fandom, the fact that Fraser Has Two Rays has caused both the Ray Wars and the occasional threesome.
Due to huge amounts of subtext between everyone involved, there is a small subset of the (already small) EUReKA fandom that ships Jack Carter/Nathan Stark/Allison Blake. Allison and Nathan used to be married, Jack and Allison have had UST since the very first episode and were married in an alternate universe, and Jack and Nathan have epic Foe Yay. Nathan's death seems to have merely spawned a host of resurrection fics, rather than stop the shipping.
In a one-episode guest spot, Nathan spent twelve hours in-canon analyzing Allison and Jack's relationship, tormenting Jack with his and Allison's romantic past, walking around without a shirt on, and implying the size of the...shoes that Jack must fill. In turn, Jack had quite a few jealous and Stupid Sexy Flanders moments, but slipped in there, "Still. It's really great seeing you again," implying that they're not all 'Foe' on the 'Yay'. Add to that the knowledge that it's all in Jack's mental projection of how he remembers Nathan... and that Nathan was crucial in resolving Jack's issues and getting him together with Allison...
The Doctor Who fanfic "Come And Knock On Our Door" by Maribor (the title is an homage to Three's Company) features 11/Amy/Rory. In the first chapter Amy attempts to seduce the Doctor. When he protests that she's married, she answers, "We talked. He's on board." Maribor's "Epistolary", which follows Amy and Rory's lives after they were stranded in the past after "The Angels Take Manhattan", implies an 11/Amy/Rory relationship existed as well in flashback scenes.
Film
Sort of the whole idea of Jim/Plato/Judy in Rebel Without a Cause—they're sort of a weird blend of a threesome and a family unit.
McCoy often becomes a part of this arrangment too.
For the prequel Star Wars, Obi-Wan/Anakin/Padme, totally. Luke/Han/Leia, before... youknow.
The Reveal hasn't stopped a lot of fans from continuing to ship that particualar OT 3
In the climax of Chasing Amy: This is actually proposed by the main character to his two possible love interests. One nervously acquiesces, but the other blows up at him, and all three part ways.
Since the Holmes/Watson/Mary love triangle is pretty much canon in the 2009 film, it's no surprise some fans have taken to OT3ing them. The three of them do seem quite cozily domestic with each other in that last scene...
Irene is sometimes added to the mix as well.
Dodgeball throws in a love triangle in the last 2 minutes - and promptly goes with this solution.
In Corpse Bride, Victor ends up learning to love Emily enough to want to join her in the Afterlife after believing his chances with Victoria are over. And Emily loves Victor enough to step aside and push him back to Victoria (who dearly loved Victor to begin with). ...Oh well, there's always the Afterlife.
Arthur/Ariadne/Eames is quite popular in the Inception fandom.
As is Arthur/Ariadne/Cobb.
Cobb/Mal/anyone isn't the most popular combination, but it makes a lot of sense, metaphorically speaking, as Mal exists only inside Cobb's head at this point.
There's definitely some kind of vibe going on between Sophie, Nathan and Stingo in Sophie's Choice
If you looked up OT3 in the dictionary, you'd find this picture◊ of Don, Kathy, and Cosmo laughing on the couch at the end of "Good Morning".
The Dutch movie Liever Verliefd ("Preferably In Love") has the main character marrying both of her love interests at the end.
This shows up in The Last Temptation of Christ, of all things. In the movie, following Mary Magdalene's death, Jesus is depicted as moving in with the sisters of Lazarus and having many children with them. Of course, this turns out to be a temptful delusion by Satan.
Similarly, those wanting to run with the Steve/Tony Ho Yay but not wanting to break up Tony/Pepper go with Pepper/Steve/Tony and those who want Tony/Bruce go with Tony/Bruce/Pepper.
Others solve the problem by the simple expedient of letting the entire cast of the Marvel Cinematic Universe have a wild orgy.
Flynn/Lora/Alan is a Running Gag in TRON fandom, backed up by the fact Lora dated Flynn before marrying Alan. However, few are actually brave enough to write it.
James and Lily, being kind of important as a couple, are often paired with Lupin, Severus, or Sirius depending on the author's favorite Seventies era student.
James/Sirius/Lupin is also quite popular.
And there's always Remus/Sirius/Tonks, with the added bonus that Sirius and Tonks are cousins.
Lunar Harmony (Harry/Hermione/Luna) has a relatively small but dedicated fanbase.
Hermione/Draco/Blaise has become mildly popular in smutfics.
Hermione/Snape/Lucius seems to become more popular by the day. Who wouldn't want two sexy older men?
As have Bellatrix/Lucius/Narcissa and Katie/Alicia/Angelina
Harry/Hermionie/Draco combines three of the most popular ships (Harry/Hermionie, Drarry, Dramionie)
Or Arwen/Aragorn/Éowyn. Throw Faramir in the mess for extra points.
Don't forget Éomer/Aragorn/Faramir!
Or Legolas/Aragorn/Arwen, or even Aragorn/Arwen/Frodo.
Legolas/Aragorn/Éomer, Aragorn/Faramir/Boromir and OT4 Sam/Frodo/Merry/Pippin.
Rosie/Sam/Frodo isn't as popular, probably because Rosie barely appears in either the books or movies, but it's out there. Considering that Rosie and Sam kept taking care of Frodo after they got married, this one works really nicely.
The Mists Of Avalon also uses it. They try an actual threesome once. It doesn't work out.
Warrior Cats just asks for these with its Love Dodecahedron nature. Firestar/Sandstorm/Spottedleaf, Crowfeather/Feathertail/Leafpool, Squirrelflight/Ashfur/Brambleclaw, and Graystripe/Silverstream all solve a lot of problems. This is made even better when you take into account that some cats have had three love interests, thus creating the OT4s Firestar/Spottedleaf/Cinderpelt/Sandstorm, Squirrelflight/Ashfur/Brambleclaw/Stormfur, and Crowfeather/Feathertail/Leafpool/Nightcloud.
Honor Harrington has this as the canon solution for the heroine's romantic woes. Although in her case the other woman is crippled, but has said if she was not...
The House of Night series by PC and Kristin Cast has many OT3s (or OT4s...or 5s...) involving Zoey, usually some sort of combination out of Zoey/Erik/Heath/Stark (and sometimes Loren). Zoey/Heath/Stark was actually canon for a while.
Laurence/Granby/Tharkay in the Temeraire series is becoming quite popular.
Jon/Tyrion/Dany also has its followers, and is not too unlikely, considering that as a Targaryen Queen, Dany would likely be allowed to take multiple husbands. The fact that Dany might be Jon's aunt doesn't really seem to matter either, since again, Targaryen.
Wicked Lovely: Irial has loved Niall for centuries, and Niall still has feelings for Irial, despite their issues. However, Niall is also in love with Leslie, who loves him back but also had a fling with Irial during their Ink Echange. The solution to their "No, you two are perfect for eachother. Without me." thing? Threesome! It's also canon after a point in the series, though.
Irial: Because you need eachother. Niall: And you. Irial: Love works like that. Niall: ...It does.
Depending how it's viewed, the end solution to the Anubis/Sadie/Walt triangle could be considered a canon version of this.
A canonical one was in Wicked. Frex/Melena/Turtle Heart.
Water for Elephants: August/Marlena/Jacob definitely has some appeal. This will really take off once the movie comes out, since it stars Robert Pattinson, Christoph Waltz and Reese Witherspoon. Curiously similar to Sophie's Choice, since both older males have volatile tempers due to mental health problems, both younger men are sort of 'adopted' into an established marriage and both younger men end up sleeping with the woman.
Twilight gives us, obviously enough, Edward/Jacob/Bella. Given the Internet's dislike for any major character other than Jacob, this is amazing. But even in the first book, there's Ho Yay between Edward and Jacob. Then Breaking Dawn gave us Edward telling Bella she could have as many kids as she wanted with Jacob
There’s also the relatively popular Alice/Bella/Edward, Alice/Edward/Jasper and Edward/Bella/Carlisle.
It’s weird that there isn’t more Edward/Bella/Rosalie, given that Carlisle and Esme initially hoped to set Edward up with her, and the way Rosalie and Bella get quite close in Breaking Dawn.
From the wolves the most popular ones are Jacob/Embry/Quil and Jacob/Seth/Leah.
The Star Wars Expanded Universe gives us Jaina/Zekk/Jag. It's actually canon. On the "coulda shoulda woulda" side of things, Ben/Taryn/Trista (hinted at), Zekk/Taryn/Trista (also hinted at), Ben/Jacen/Tahiri (we could ship...on Ship!), Luke/Corran/Mirax (Luke even hinted as such in I, Jedi.), Luke/Corran/Mara (more Stackpole subtext, which is not very subtextual unless it's Ho Yay but a lot more subtextual than his spiritual heir Troy Denning), Ahsoka/a clone/another clone. Not really loli, since they're younger than her!
Very nearly canon in The Lions Of Al-Rassan with Ammar/Jehane/Rodrigo.
It might well have solved some fairly major problems if the setting of Gone with the Wind had been one in which an Ashley/Melanie/Scarlett One True Threesome had been a possible option.
Canon for the lead three characters in the Bold As Love series.
The GONE series has the ''Devils trinity'' which consists of Drake/Caine/Diana. This ship is popular, mainly because these three characters have many conflicting (and popular) ships, like ''Cake'' (Drake/Caine), ''Caina'' (Caine/Diana, actually canon) and ''Driana'' (Drake/Diana).
Is often parodied seeing as all three of them seem to hate each other at least 50% of the time in canon. That's not to say there isn't a lot of ship teases between them...
In Les Misérables fandom, there is the technically canon threesome of Joly/Musichetta/Bossuet, and in some fanfics the main love triangle is solved by shipping Cosette/Marius/Eponine.
Doctor (usually 9)/Rose/Jack Harkness. This came as close to canon as possible for a kids' show: Jack and Rose are both in love with the Doctor, Jack and Rose also fancy each other, the Doctor loves Rose (but Cannot Spit It Out and never states whether it's romantic love or something else) and merrily flirts with Jack, and all three pairings get romantic and sweet kissing scenes. Jack in particular just flat-out ships himself with the other two.
Blink and you'll miss it but there's also the possibility for Ten/Martha/Shakespeare in "The Shakespeare Code" as the good Bard openly flirts with both of them
Doctor/Master/Rani. It is delicious Foe Yay, you must ship it.
Two/Jamie/Zoe, as seen here◊. Three characters, one hairdo.
There's an RL platonic element to this one. The three actors were quite close and, according to Frazer Hines, even made some of the other actors jealous on the set of "The Five Doctors". Hines also stated that none of the constant hugging was scripted — it just came natural.
Eleven/Amy/Rory (as pictured above) is a bit of a variation. Rory is completely Amy-sexual, Amy is in love with Rory but obsessed with the Doctor and wants to fit them both into her love life somehow, and the Doctor consciously represses his attraction to Amy (cf. "Amy's Choice"), but quickly grows to really like Rory with heaps and heaps of Ho Yay. Fandom seems to have embraced it big time. Given that their idea of a honeymoon is to run off with the Doctor in the TARDIS... And they look gorgeous.◊
Though now that Amy and Rory have been revealed to be the Doctor's mother- and father-in-law, this went straight into the direction of averted trope... until Eleven (being Eleven) decided that this was in no way a reason to stop randomly snogging Rory.
It's just one episode, but Eleven/Amy/Vincent is pretty strongly hinted, since Vincent has strong chemistry with both the Doctor and Amy, to the point of the three lying in the grass stargazing, all holding hands. What with the Doctor's ginger fetish and Vincent asking Amy to marry him, things might have ended very differently had there been no Rory.
Five/Tegan/Turlough and Five/Tegan/Nyssa.
Five/Adric/Nyssa/Tegan could almost be an OT 4 (as well as picking any three of them to make an OT 3).
Going back to the Tenth Doctor again, Ten/Rose/Handy (Handy here being the fan nickname for 10.5, the duplicate Doctor that grew from the Doctor's severed hand. Long story. Really long story.) Some people don't think it's quite fair that Rose should have to choose one of them and leave the other pining for her until the end of his days. Or something equally dramatic.
Quite a few people seem to like Barbara/lan/Susan.
To go with the infamous solution to the class Love Triangle, you have Jack who's in a canonical relationship with Ianto, but has UST with Gwen. Gwen returns the UST and has shown quite an interest in Jack and Ianto's sexytimes. At one point, Jack even offhandedly invited Gwen to join in on one of their sessions of "naked hide-and-seek!"
There's also people who think they should just have a five-way aka a Torchwood orgy.
While it might be slightly abusive, fucked up and Wilson/Cuddy tend to also act like House's parents, House/Wilson/Cuddy is still a firm favourite in the House fandom.
Until Amber's death, more than a few fans were getting into Amber/Wilson/House as well.
There is no shortage of House/Wilson/Stacy, Chase/Cameron/Thirteen, and Chase/Foreman/Thirteen.
Despite more than a few common non-romantic trios throughout the years, OT3 shipping really didn't hit its stride in Power Rangers until Dino Thunder, in which some fans started trying to pair Conner/Kira/Ethan. Though before tat, Wes/Jen/Eric was pretty popular.
Season four has given us the carny OT3 of Samuel/Lydia/Edgar.
Bernard/Fran/Manny in Black Books. Three co-dependent friends, both of the guys being quite confused about their sexuality but usually too drunk to care, and most of the plot revolving around extreme boredom. The fanfic writes itself.
And then too, Seven/Janeway/Chakotay takes the sexual tension established between Janeway and Chakotay throughout the series, takes the mentor/student (or mother/daughter) dynamic between Seven and Janeway to the next level, and reluctantly lets in the out-of-left-field pairing of Seven and Chakotay we had tacked on to the last episode of the series.
Marshall/Lily/Ted is semi-canon, minus the sex, most explicitly in "Moving Day", when Marshall and Lily are thrown completely out of whack when Ted moves out (the three have lived together since their freshman year of college). Lily even tells Marshall "if we want to get married, we have to marry Ted too!", and when Ted comes back to them at the end of the episode, they enfold him in a two-sided embrace that's very blatantly played as a romantic make-up.
And Barney with all the girls because he's Barney.
The Top Gear fandom has a lot of OT3 fic. Sometimes the third party is a car.
Despite the massive Shipping Wars, quite some Buffy the Vampire Slayer fans are followers of Angel/Buffy/Spike. This is even acknowledged in one Joss Whedon-written issue of the canonical Buffy Season 8 comic book, where it is revealed that Buffy's sexual fantasy is a threesome between her, Angel and Spike. Even more kinky, she wears a nurse uniform. And they're chained to her.
Throw Dru in for super awesome PTB points.
Oz/Willow/Tara could solve plenty of problems.
Buffy/Willow/Xander had quite a number of supporters in the early seasons.
Warren / Andrew / Jonathan isn't that uncommon, you know...
Buffy/Willow/Tara has quite a few supporters too, especially when you consider that Willow & Tara moved in to the Summers' house and were already essentially honorary big sisters to Dawn anyway.
Given Dru's happiness at the three of them being together again and Angelus' implied past relationship with both Dru and Spike, Angelus/Drusilla/Spike is the obvious conclusion. Darla would also be included, but no-one likes her.
Over at sister series Angel, the unexpected family-like closeness among the original cast (Angel, Cordy, and Doyle) won many fans, some of whom 'shipped the whole threesome. This went double for Angel, Cordy, and Wes — after fans forgave the latter for replacing Doyle, that is. There's also Gunn/Fred/Wes, to reconcile the shipwars.
Big Bad Jasmine even hinted to Gunn and Wes that they should do this in order to stop fighting over Fred.
In Due South fandom, the fact that Fraser Has Two Rays has caused both the Ray Wars and the occasional threesome.
Due to huge amounts of subtext between everyone involved, there is a small subset of the (already small) EUReKA fandom that ships Jack Carter/Nathan Stark/Allison Blake. Allison and Nathan used to be married, Jack and Allison have had UST since the very first episode and were married in an alternate universe, and Jack and Nathan have epic Foe Yay. Nathan's death seems to have merely spawned a host of resurrection fics, rather than stop the shipping.
In a one-episode guest spot, Nathan spent twelve hours in-canon analyzing Allison and Jack's relationship, tormenting Jack with his and Allison's romantic past, walking around without a shirt on, and implying the size of the...shoes that Jack must fill. In turn, Jack had quite a few jealous and Stupid Sexy Flanders moments, but slipped in there, "Still. It's really great seeing you again," implying that they're not all 'Foe' on the 'Yay'. Add to that the knowledge that it's all in Jack's mental projection of how he remembers Nathan... and that Nathan was crucial in resolving Jack's issues and getting him together with Allison...
In the Leverage fandom, the solution to the Parker/Hardison vs. Eliot/Hardison seems to have mostly resolved itself in this fashion.
Fans of the Ho Yay in WhiteCollar don't seem to have a problem including Peter's wife Elizabeth in the fun (probably because she is so awesome, and because she and Peter are so adorably Happily Married). It also helps that Elizabeth, while being fiercely devoted to Peter, isn't shy about being a fan of Neal's blue eyes and charm (in an "I'll stick to my diet, but still enjoy looking at the menu" way.)
To be fair, it seems like the promo monkeys◊ are shipping Neal/Peter/Elizabeth. (The promo picture shows Peter kissing El while handcuffed to Neal and all three actors' unbelievable chemistry.)
In the Supernatural fandom, some people ship Sam/Dean (Wincest), some people ship Dean/Castiel (Destiel), a rare few ship Sam/Castiel (Sassy), and some people say "Forget it" and ship Sam/Dean/Castiel (Wincestiel/Team Free Will).
Every male character has been shipped in a threesome. [[Rule34 No exceptions]]. And given the fanbase,foursomes and fivesomes aren't that uncommon. Other popular shipmates include Gabriel, Benny, Luci, and Meg.
There's also Puckleberry Quinn for those more fond of Les Yay than Ho Yay
Kevin McHale, Naya Rivera, and Heather Morris have all said that they would love to see Artie, Santana, and Brittany get together.
Sebklaine (Sebastian/Blaine/Kurt) and Klaineofsky (Dave/Kurt/Blaine) are also known to occur in fanfics
The most popular and lampshaded in the series however, is The Unholy Trinity, comprised of Quinn, Santana and Britnney. They both used to hang out almost exclusively as a three and were considered best friends in the first 8 episodes (Before shit ensued). What makes this so awesome is that it's plausible. Santana is a lesbian, Brittney is Bisexual, and Quinn has had a fair bit of Les Yay in the past, what with her drawing pornographic pictures of girls on the bathroom walls and drawing pictures of girls with hearts around them.
Life On Mars/Ashes to Ashes: Oh, so many. Sam/Gene/Annie has been around nearly from the beginning, and A2A has really been pushing the Ray/Chris/Shaz and, let's face it, Gene/Alex/Keats lately.
3.07 has Keats lampshading the Gene/Alex/Keats overtones:
"Do you think this is a game, Alex? A silly, flirty little game being played out by you, me, and Hunt?"
However with the American version in reality Annie is Sam's stepmother, and Gene Hunt is his father making this pairing a double-sided Oedipus Complex
Big Time Rush, most popular all four of the main boys, James/Logan/Kendall/Carlos, but you'll find pretty much any threesome combination if you look hard enough, sometimes even with one of the girls thrown in.
James/Logan/Camille is incredibly popular right now, especially considering how Logan orchestrated James and Camille's dress/tux trade in the recent prom episode.
In season 5, Jay/Alex/Paige. And Paige/Alex/Hazel.
In season 6, Jimmy/Spinner/Ash. And Sean/Emma/Peter. And Sean/Emma/Manny. Just...Sean in general.
In Season 8-9, Spinner/Holly J/Jay. Or Spinner/Jane/Holly J. Or Declan/Jane/Holly J. And Peter/Riley/Anya has some following (Peter/Riley/Anyone has some fan following).
In Season 10, Eli/Clare/Fitz or Eli/Clare/Adam... or Eli/Clare/Anyone really. Eclare has a lot of following. While Peter is out of the series, Riley/Anya/Zane and Riley/Anya/Dr. Chris have some fans making eyes.
The whole time Sean and Ellie are together, Craig and Ash either hate each other or are "best friends", and Craig/Ellie attracts a huge Hatedom, but the Fun with Acronyms of Ashley/Craig/Ellie/Sean is worth it.
Tin Man has been known to take this route with the three human party members.
Damon/Elena/Stefan in The Vampire Diaries. The primary proponent of this ship appears to be whoever does the show's promo pictures, which frequently portray the situation as less "I'm dating a vampire, and have a certain amount of sexual tension with his brother, which makes things a bit awkward" and more "Yeah, I'm totally doing them both, probably at the same time." This?◊ Not happening.
It's further complicated by the part where Elena is the identical descendant of the vampire Katherine, who was - though not actually at exactly the same time, since she was playing them against each other. It was a popular fan theory that Katherine's goal was to do this... though Katherine later claimed that she was only really ever interested in Stefan. She may have been lying.
Merlin/Arthur/Gwen from Merlin is practically canon. With the three of them destined to rule Camelot together there is an outrageous amount of shipping going on, fuelled entirely by the show itself. A recent dream sequence showed Arthur crowning Guinevere Queen of Camelot whilst Merlin watches with a look of abject adoration on his face. Heck, most of their scenes together involve them staring at each other lovingly.
Oh, look. Gwen and Arthur have gone on a private outing to celebrate their anniversary. Wait, is that ...? Yes, they brought Merlin along on their anniversary date. Clearly, he's also married to them both.
Then there's the popular OT4 (Merlin/Arthur/Gwen/Morgana), sometimes expanded to OT5 by adding Lancelot.
Merlin/Arthur/Gwaine is also quite popular.
More recently, the Knights of the Round Table OT 7, consisting of Arthur, Merlin, Gwaine, Lancelot, Leon, Percival, Elyan and possibly Gwen.
Gwaine/Percival/Leon and their actors seem to become popular due to interviews and twitter.
JD/Turk/Carla in Scrubs. It's almost adressed in the show itself by Carla.
When Turk is about to ask JD to be his best man at their wedding, JD somehow gets the idea they're about to suggest a threesome. His Inner Monologue doesn't sound too averse to the idea...
"Oh my god, would I have a threesome with Turk and Carla? Well, it's certainly flattering and I don't think they'd tell anyone..."
Horatio Hornblower has Horatio/Archie/Bush in "Mutiny" and "Retribution". Archie Kennedy having been Horatio's best friend for the first series, and Mr Bush being his canonical best friend who actually appears in the books. While the Hornblower fandom is not known for a lot of animosity between ships (pun intended) any conflict between Archie/Horatio and Bush/Horatio can be solved with this.
Sheridan, Delenn and Lennier looked sort of like this in Babylon 5 for a long time but in the end it didn't quite work out. Some Fan Fic writers disliked this.
Seems to be a fair amount of interest in Abby/Becker/Connor in the latest Primeval series. Helped along by the fact that these are the only remaining team members from the previous series (aside from Lester.) Oh, and they shared a rather emotional three-way hug in the opening episode of series 4. Plus, they're all awfully pretty...
Mitchell/Annie/George on the UK version of Being Human, particularly in the early days. Occasionally made into an OT4 with the inclusion of Nina.
Tom, Hal and Alex have this vibe going on in series 5. There's some romantic tension between Hal and Alex as well as lots of Ho Yay between Hal and Tom.
Sookie/Eric/Bill in True Blood was shipped way before it became canon.
Troy/Abed/Annie seems pretty popular in Community fandom. This may be because shipping Annie with either of the two guys singularly is itself a very popular option while in canon Troy and Abed have a very intensely close Ho Yay-laden bromance with each other (which, naturally, has led to shipping in and of itself), and this is a nice third option where everyone wins.
Since Lauren can't sufficiently feed Bo, this is a ligitimate alternative.
And now with the addition of Tamsin there might actually be a Lauren/Bo/Tamsin one coming.
Game of Thrones (the series, but not the books) looked like it was heading in the direction of canonical Margaery/Renly/Loras, suggested by Margaery herself in response to Renly's inability to consummate their marriage. Although Margaery and Loras are brother and sister, the addition of a third, non-related person means it's still a relatively wholesome arrangement by the series' standards. Then Renly died
Mitch/Alice/Richie from the accurately named Threesome. The show plays up the relationship between all three characters with each other as romantic, so it is near impossible not to ship them as an OT 3. Plus, the whole point of the show is that the group have a threesome in the pilot.
Donna/Harvey/Mike on Suits, while not as popular as Harvey/Mike, gained a lot of popularity in season 2, with Hardman's arc. Donna and Mike both have Undying Loyalty to Harvey; Donna is Mike and Harvey's Secret Keeper, and when Harvey's position is at risk, Mike and Donna work together to get help him win. When Donna gets fired, Mike and Harvey pull together to get her back.
"Pretty Little Liars" has has a couple of O T3s but Spaily has very recently grown in popularity. Combining the canon Emily/Paige (Paily) with the fanon Emily/Spencer (Spemily) and working off of the tension in Spencer/Paige interactions.
Other O T3s include Lucas/Hanna/Caleb and Wren/Hanna/Spencer.
Canonically subverted in the second season of American Horror Story with Kit/Alma/Grace. They set up housekeeping together but it does not end well, especially for Grace.
Lassie/Shawn/Jules seems to be pretty popular amongst Psych fans, sometimes with Gus thrown in. It helps that this particular arrangement would very much not be out of character for Shawn to support.
Skins has Freddie/Effy/Cook - sometimes even outright implied in the show.
Music
Todd in the Shadows ships Ke$ha with the two members of 3Oh!3. He just hopes their union never produces any children.
Newspaper Comics
Some fans of Peanuts have suggested this as a way to reseove the Marcie/Charlie Brown/Peppermint Patty triangle.
Oh, let's just keep it concise and sum up as follows: Greek gods were shapechangers, Gender Benders, could and did screw just about anything that moved, and a few things that didn't.
For added weirdness, remember Tiresias the prophet? He lived as both sexes (male and female) sequentially due to a bet Hera made with Zeus.
Additionally, Phantom/Christine/Meg in the fanfic/fandom zone. Two girls? Joys of the flesh indeed. I don't hear the Phantom protesting too hard...
The Claude/Berger/Sheila threesome in the musical Hair is pretty much canon, leading to a three-way embrace.
Jeanie: . . . Sheila's hung up on Berger, Berger is hung up everywhere. Claude is hung up on a cross over Sheila and Berger.
In Wicked a great many problems would be solved by Elphaba/G(a)linda/Fiyero.
Or Boq/Nessa/Galinda.
The Noël Coward play Design for Living explores every possible combination of playwright Leo, artist Otto, and decorator Gilda before finally having them inform her husband Ernest that his services are no longer required, as the three of them intend to live in what Ernest decries as a "disgusting three-sided erotic hodgepodge."
The Merchant of Venice has a lot of subtext for Portia/Bassanio/Antonio. Usually in performance the implication is that Antonio is getting left out in the cold, but the odd production has run with these implications in staging the ending.
Video Games
Yukimura/Mitsunari/Kanetsugu from Samurai Warriors 2, to the point that KOEI even released an image song featuring the three.
Yukimura/Kunoichi/Kai was introduced in SW 3. Yukimura/Kunoichi/Ina as well. Hell, pretty much every woman Yukimura meets falls for him and every guy admires him. It's a shame there won't be any canon relationships (threesome or otherwise) due to Yukimura's historical status as a Chaste Hero.
There's also Muneshige/Ginchiyo/Yoshihiro and Hideyoshi/Nene/Oichi ( this being Hideyoshi's ideal outcome). Given the series' Lots And Lots Of Characters there are a lot of these.
Final Fantasy IV gave us the archetypal Final Fantasy example in Cecil/Rosa/Kain. It's always had a small but loyal fandom, but the DS remake and official sequel has made it skyrocket in popularity of late.
Final Fantasy VI gave us Terra/Celes/any human male. And Locke/Edgar/Terra; the two even swoon over her when they find out she can use magic. And, as an artifact from the Star WarsShout Out, Biggs/Wedge/Terra. And Locke/Celes/Setzer.
There's a certain amount of fandom for Aerith/Tifa/Cloud, Zack/Sephiroth/Cloud (referred to as Omnislash), and Cid/Vincent/Shera hidden amongst the fanworks of Final Fantasy VII.
As well as Zack/Aerith/Cloud, to a certain extent.
For greater fun, there's always Zack/Aerith/Sephiroth/Cloud.
Don't forget RedXIII/Tifa/Yuffie.
And Crisis Core provided yet another OT3 consisting of Sephiroth/Genesis/Angeal, which in turn spawned Sephiroth/Genesis/Angeal/Zack/Cloud. Yes, an OT5.
Though it doesn't enjoy the popularity of some other OT3s due to the vocal Hatedom for Rinoa, Final Fantasy VIII gives us Squall/Rinoa/Seifer.
Final Fantasy X has quite big slice of fanbase what supports Jecht / Auron / Braska. Three ''very''handsome men, who would've given their lives for each other. And - in fact -, they did Jecht and Braska gave their lives so Auron (and others) could be save and Auron got so messed up from losing them so he tried to avenge them in mad rage, getting himself killed.
Yuna/Rikku/Tidus is also quite possible. Technically Yuna and Rikku are cousins, but since Rikku's brother Brother has a crush on Yuna apparently this doesn't really matter.
In the Final Fantasy XIII fandom there's an OT3 called "Neapolitan" with the dark-haired and dark-skinned Fang (chocolate), the pink-haired Lightning (strawberry), and Vanille (guess), which has reached explosive levels of popularity.
The Phoenix Wright: Ace Attorney kink meme has a 'polyamory' tag for just this purpose. Edgeworth/Phoenix/Maya is the most common, but then again they're also the most popular characters in the fandom anyway.
As of Investigations Yew/Faraday/Badd seems to have picked up some support.
Anyone who didn't see this in Wild ARMs 5 with Dean/Avril/Rebecca needs to have their eyes checked.
Not so unavoidable, but for the original, both Jane/Rudy/Cecilia and Rudy/Jack/Cecilia are fairly workable and turn up in fic.
The main Wild ARMs 3 cast, Virginia, Jet, and Gallows, all had too much charisma together (and too much time spent alone... in the wilderness...) to not have SOMETHING going on. (Clive would be in there too, but he's too loyal to his wife.)
Skies Of Arcadia: Vyse/Aika/Fina, with the added bonus that the game seems to lean towards this being canon. Both Aika and Fina seem to be aware that the other likes Vyse, but neither seem to mind.
Kingdom Hearts has this trope as something of a theme, with many of the characters fitting a general combination of three major virtues represented by the Sword, Staff, and Shield. Though it's treated as utterly non-sexual, pretty much every major character belongs to a trinity: Sora/Donald/Goofy (though it takes a very special type of fan to ship them), Sora/Riku/Kairi (in their case, a Het-versus-slashshipwar resolution), Roxas/Xion/Axel (fans love the Accidental Innuendo of the expression "to have ice cream"), and (presumably) Ven/Aqua/Terra... the list goes on.
There's also Roxas/Axel/Namine by association. Roxas and Axel spend a a lot of time together in the games (and manga), Axel and Namine spend a lot of time together in the manga (though there are moments in KH 2 FM+ ), and then Namine and Roxas spend some time together in the beginning and ending of the games (and the manga).
Also popular are Sora/Axel/Roxas, especially thanks to the reveal that both Sora and Roxas made Axel feel like he had a heart. Ignore the fact that Roxas and Sora are essentially the same being.
Hell, even Tidus/Waka/Selphie exists in the purely Kingdom Hearts Universe, and they're so minor that only one of them has a speaking part in five games.
Then there's a small following of Xemnas/Saďx/Kingdom Hearts. Make of that what you will.
Eirika/Ephraim/Lyon is practically canon, or at least it would have been had it not been for, well, you know... That said, considering this is Fire Emblem, there are likely quite a lot of these across the different games.
You could make a case for Innes's sister Tana as well. On one hand, she's kind of a Tsundere for the mostly oblivious Ephraim. On the other, her and Eirika are a textbook case of Romantic Two-Girl Friendship.
Tibarn/Reyson/Naesala is common among the Tellius fandom, as is Lucia/Elincia/Geoffrey.
Ryudo/Elena/Millenia, from Grandia II, with the bonus that it's actually canon.
Disgaea: Hour of Darkness: Laharl/Flonne/Etna, so obvious it hurts. Lampshaded when Laharl pretends to be Mao's father in Disgaea 3, and Etna and Flonne have some trouble over just who's supposed to be the mother.
A few pop up in Tsukihime and Melty Blood. Shiki/Arcueid/Ciel and Shiki/Hisui/Kohaku are the most common. There have been fandoms of Shiki/Akiha/Sion; and with newcomer Riesbyfe to the Back Alley Alliance we have Satsuki/Sion/Riesbyfe.
In Tsukihime's successor, Fate/stay night, the Saber/Shirou/Rin triad has many fans. This is naturally bolstered by the infamous threesome in Saber's route, although it's more business than pleasure. Most of the shipping focuses on the good ending of Rin's route, in which Rin manages to keep Saber around post-Grail War, and implies that continuing to do so will require lots of... input from Shirou. If one is building an OT3, it helps to have one of those involved be both admittedly bisexual and not the sort to disguise her interest.
Another popular triple 'ship is Sakura/Shirou/Rider. This stems from one of the bonus scenes in Fate/hollow ataraxia, in which they have a threesome, but it turns out to be an illusion Rider's creating to get mana from Shirou. This scene is why Rider gets counted in Shirou's Unwanted Harem, and since she's a fan favourite, her relationships with Shirou and Sakura get lots of attention in doujins.
The English Dating SimSummer Session features a three-person relationship as one of the canon endings; in fact, it's impossible to get that particular girl without sharing her with your roommate. It's unclear whether the two male characters are actually involved with each other or just sharing the girl in a V.
School Days: One of the endings involve Sekai and Kotonoha actually sorting their differences more or less amiably, and then deciding to share Makoto's affections. To say the least, they do not reach such an agreement in the anime (or the manga).
In Cross Days Manga however, both are aware of other dating Makoto, and seem fine with it. Kotonoha goes as far as call Makoto AND Sekai her "extremely important and beloved people".
The Tales Of Graces fandom has the Asbel/Sophie/Richard 'ship as well as the OT4 of Asbel/Sophie/Richard/Lambda.
She looks twelve for most of the game. She ages up at the end though. And, well isn't it kind of weirder that she's technically...y'know...a 1000-year-old robot?
Pick a Castlevania with Two Guys and a Girl and there are shippers for it. Most frequently found with the Metroidvania installments of the series, since that trope is most common in the IGA penned games, but it cropped up as early as Dracula's Curse, with Trevor Belmont and his various companions. Richter/Alucard/Maria is easily the most popular one, though Alucard/Isaac/Hector has a small but loyal following in the Japanese fandom.
Possibly canon for one of the endings of Date Warp with Janet/Linds/Rafael.
Super Mario RPG has a bit where you get to pick up all of Peach's gear. You get a kiss either way. Pick up it all, and you'll get the only heterosexual option. Bowser and Booster, on the other hand... You can also get Mario/Bowser and Booster/Peach or Mario/Booster and Bowser/Peach. Finally, the only one relevant here you can get Mario/Bowser/Booster, and Peach...presumably she's a Yaoi Fangirl.
Persona 3: Main character/Aigis/Elizabeth, as the only known multi-Persona-users within the timeframe of that game, and given that Aigis arguably becomes Elizabeth's only other significant social contact from Earth aside from their mutual romantic interest. Mitsuru/Akihiko/Shinjiro is also popular given their shared history.
The introduction of a female protagonist option in Persona 3 Portable resulted in Shinjiro/FeMC/Akihiko, given Shinjiro and Akihiko's close relationship and the fact that both of them are possible love interests for the protagonist at different points in the game. This is sometimes expanded to include Mitsuru as well.
This has been done a lot in the Star Fox fandom, and since most of the regular characters are male, most combinations are gay. Fox/Falco/Peppy has been done, as have Wolf/Leon/Panther and James/Peppy/Wolf.
Command & Conquer: Red Alert 2 and 3: Eva or Tanya, and the Allied Commander. At the end of either game (especially 3), the Allied cutscene shows them making their mutual plays for him. So, given that the endings are left open-ended, guess what a lot of fans say he should do?
Especially if you're a female Commander. Les Yay for all!
One fairly obvious OT3 in Suikoden V is Frey/Lymsleia/Lyon, due to the blatant subtext heaped on the former two and the text about Lyon's feelings for Frey. This is sometimes expanded to include Miakis as well, since she and Lymsleia have their own helping of Les Yay.
Assassin's Creed has Altair/Malik/Kadar and Altair/Malik/Maria as the most common ones (the latter mainly because it neatly combines the most popular fan pairing and the canon pairing), with some Desmond/Lucy/Shaun/Rebecca and Ezio/Leonardo/(character of your choice) on occasion.
And a suprising number of Ezio/Federico/Vieri fics.
Phantasmagoria 2 is rare in that it has a bisexual hero (Curtis) wishing he could run off and live as a threesome with his girlfriend Jocilyn and his doomed gay coworker Trevor (both of whom are in love with him). Also, at one point Curtis was dating both Jocilyn and his BSDM loving coworker Therese, (but only Therese was okay with the idea of their having an open relationship.)
In Dragon Age: Origins, it's usually Isabela as the lynchpin, since Three-Way Sex is canonically possible with her in both games she appears in. Hawke (of either gender)/Fenris/Isabela has a sizable shipping fanbase. Hawke/Anders/Justice seems to be rather popular among Dragon Age fans. This makes for an interesting situation with three people and only two bodies involved. It's also held up as a Lighter and Softer option for that Sadistic Choice Morrigan throws a female Gray Warden dating Alistar.
Slightly less common in Mass Effect fandom, but there's shippers for a few, Shepard/Garrus/Tali and Shepard/Garrus/Thane being two of the more notable ones. The former has experienced an uptick in popularity since the third game made Garrus/Tali canon if neither are romanced.
In Mass Effect there is also Team Milkyway consisting of Shepard/Kaidan/Ashley. It saw a particular spike in popularity when Kaidan's bisexuality was revealed, since that meant the only thing theoretically preventing it is the fact that there's no way for all three of them to live through the series.
Jade Empire is notable in that the OT3 (male PC/Silk Fox/Dawn Star) can actually become canon.
Before those, it was possible in Neverwinter Nights: Hordes of the Underdark with male PC/Nathyrra/Aribeth, which could also become canon (with enough points in Persuade).
In Knights Ofthe Old Republic fanfic, it's not uncommon to see Female!Exile/Atton/Mical, or M!Exile/Handmaiden/Visas for the second game or F!Revan/Carth/Canderous, or F!Revan/Juhani/Bastila on the first game.
You can try it in The Old Republic, but none of your potential love interests will go for it (yet?). Of course, any romance option with Vector is going to be a crowd scene since he's tied to a Kilik hive.
Dangan Ronpa has Chihiro/Ishimaru/Mondo, charmingly called Chishimondo. There is also a canon example in the case of Naegi/Aoi/Hagakure/possibly Togami, in the bad ending at least, though oddly enough it's rare to see that in fandom.
The sequel, Super Dangan Ronpa 2, has a love triangle between Sonia, Souda, and Gundam. So, of course, some fans like to ship all three together.
ProfessorLaytonAndTheMiracleMask has Angela/Randall/Henry. Angela/Randall is canon, Angela and Henry are married for over a decade, and Henry shows astounding devotion towards Randall even after his death. On the less plausible side, there is Layton/ Claire/Dimitri from Unwound Future
Webcomics
Niels by Humon has the threesome of Duncan/Niels/Natalie (Natalie and Niels can switch depending on what mood he's in). This was all Niels' fault too.
While Gilgamesh Wulfenbach actually proposed to Agatha at one point (more politically than romantically) and is in the position usually held by the Designated Love Interest, the Girl Genius fandom seems to support Agatha/Gil/Tarvek. The fact that the Cinderella parody done in one of the interludes ended with Agatha-as-Cinderella marrying both Gil and Tarvek indicates that the writers don't think it's in the realm of the impossible either.
It helps that Agatha has found that she does in fact own a Harem (albeit empty), which is where that conversation took place. And they were under the influence of a mood-altering carnivorous plant.
In the last panel of this page, Tarvek's expression and dialog point to some genuine feelings towards Gil, though whether this is friendship or something deeper is still open for interpretation.
The Insecticomics has a lot of this, usually in the form of open relationships.
Davan/Vanessa/PeeJee, anyone? Vanessa has even stated that she would not be averse to such a set up.
The author, on the other hand, is. Davan and Peejee will never be a canon item. That doesn't mean we can't dream, though…
Ash/Emily/Missi would solve a lot of problems, now wouldn't it?
When pigs fly with the aid of neither magic, science, nor Magic Science.
The Flash Forward strips in the Space Arc of Arthur, King of Time and Space suggest that the Arthur/Guenevere/Lancelot triangle resolves this way in at least one timeline. Other strips hint that Arthur is well aware of the affair between his wife and his best friend in at least some of the other timelines, as well, but studiously ignores it.
City Of Reality is a literal, and deconstructedMary Sue Topia, to the point where, when a person catches their spouse cheating, the natural and normal reaction is to join in.
Questionable Content: Marten/Dora/Tai has been raised in-comic. Unfortunately, it's also been shot down - Dora may be bisexual, but she's still monogamous.
Torg's Evil Side: Take them both! Take them both to Disneyland and buy them concession stand drinks! Save the cups to sample their DNA later and create a harem of beautiful Zoë-Oasis hybrid loveslave clones!
Collar 6 gives us the canon couple of Sixx/Laura/Ginger. Made possible as all the girls were already into polyamory beforehand.
Jack/Zimmy/Gamma has popped up among certain fans of Gunnerkrigg Court after a revelation or two in the comic. Mostly for laughs, but a few not so much.
Homestuck has a couple of popular OT4s with people pairing up the groups of kids together. More traditionally, it's pretty common to see some love triangles resolved this way, creating pairings like Dave/Terezi/Karkat or Eridan/Feferi/Sollux. More unusually, because trolls in Homestuck canonically have four different kinds of romance and usually desire to have one partner fulfilling each, it's common for OT3s to involve multiple different relationship types. Using the earlier example, it's not unusual to see people shipping Eridan, Feferi, and Sollux all in red relationship (that is, a normal human romantic one), but it's arguably even more common for people to ship Eridan and Sollux both in a red relationship with Feferi but in a black relationship with each other. Needless to say, fics with lots of different pairings and multiple quadrants for each character can get very... complicated.
To note, auspistices (c3<) in troll culture are a form of three-way relationship in which one party manages an unstable or potentially socially damaging relationship between the other two. Alas, it is entirely platonic.
There appears to be a number of El Goonish Shive fans that ship Sarah with one of the Official Couples of the comic, Tedd / Grace. She has received some Ship Tease with both of them, but she is currently part of another Official Couple. However, given certain developments in the comic, she might actually be free to fulfill the wishes of the fans.
Web Original
Vocaloid shippers like to choose three of the female Japanese Vocaloids (At first mostly Vocaloids 00, 01, and one half of the reflection of 02 before Megurine Luka.), but another common OT 3 came shortly. With a third male Japanese Vocaloid, an obvious conclusion: Gakupo Kamui was tacked on to KAITO/Kagamine Len when he came onto the scene.
The Whateley Universe right now has Canon triangles of Thunderbird/Chaka/Riptide and Stalwart/Fey/Bugs and Molly/Chou/Dorjee. Fandom has wondered about whether the Ayla stories are headed for a Vox/Phase/She-Beast triangle.
The Molly/Chou/Dorjee triangle has been resolved via Polyamory, might to the delight of certain segments of the fandom, I'm sure.
The TGWTGCrack Pairing of MarzGurl/Linkara/Insano is pretty popular, and really quite sweet most of the time. Chick/Critic/ATG is popular too, although that's usually saved for kinky Dark Fic, naturally.
Spoony/Chick/Googles is canon. As is Spoony/Critic/Joe.
Despite the longstanding bad feelings that generally exist between the Zutara and Kataang camps, there has been an increase in Zuko/Katara/Aang fanworks lately.
Jet/Longshot/Smellerbee has also increased in fandom after Longshot/Smellerbee was declared canon.
Much of Sequel SeriesThe Legend of Korra fandom have already declared their support for Korra/Mako/Bolin. No, the fact that the two boys are siblings didn't even slow them down. In light of the secondary Love Triangle of Asami/Mako/Korra, and the ensuing Ship-to-Ship Combat with the Mako/Korra/Bolin shippers, some have simply decided to ship them all as a beautiful lovequadrangle, MaKorraSamiLin.
Tweek/Craig/Thomas and Kyle/Stan/Wendy are also pretty common.
Ed/Edd/Eddy, of Ed Edd N Eddy fame. I'm serious; it's out there, and gaining popularity.
Also Marie/Edd/Eddy and Kevin/Edd/Eddy.
Ben 10: Ben/Gwen/Kevin solves a fewtangled problems. If people are sympathetic to Julie, they might add her in as well.
From The Road to El Dorado, Chel/Tulio/Miguel. Considering the ending, it's almost-sorta-kinda canon!
Noah/Cody/Gwen is fairly popular for fans of Total Drama Island who can't decide which to pick. Sometimes Izzy is thrown in as well for a neat little foursome.
With the growing popularity of Noah/Izzy and the continuing presence of the Noah/Cody fanbase, several fans enjoy using Izzy/Noah/Cody instead, because, lets face it, Izzy would be happy to share.
After Gwen and Courtney became friends in Season 3, at least before I See London, there was some fan possibility of Courtney/Duncan/Gwen as a threesome.
In Toy Story 3 after Bo Peep got Put on a Bus, Woody/Buzz/Jessie became quite popular. In fact, it was even implied to be canon. Er, at least in "Spanish Mode". Woody/Bo/Buzz/Jessie has also picked up in popularity.
It may be doubtful if they would've had a good relationship, but Daria/Tom/Jane would've averted a lot of headaches from everyone involved...
Futurama's Fry/Leela/Bender would solve everything instantly. Fry would have Leela, Bender would have Fry, Fry and Bender would still be their awesome duo, and Leela and Bender would start getting along because they'd have to share Fry. Everyone would finally be happy. Which, unfortunately, never happens in Futurama.
Kim Possible: Kim/Ron/Shego would solve a lot of fighting between the K/R and Kigo shippers in the Kim Possible fandom. Of course, that generates an entirelydifferentproblem.
If one is into Incest Yay Shipping, then Princess Cadance/Shining Armor/Twilight might count too.
While both Applejack/Rarity and Applejack/Rainbow Dash are some of the most common ships, the predictable OT 3 must deal with the infeasibility of Rarity/Rainbow Dash. Few succeed.
Celestia/Twilight/Luna, if you can handle the princest.
Young Justice has a very popular Robin/Artemis/Kid Flash relationship, despite Ship-to-Ship Combat between Robin/Artemis and Artemis/Wally. The three wind up being close friends by the beginning of season two with Wally/Artemis as the Official Couple, but Robin and Artemis' interaction came off as unintentionally flirty to some viewers. The trio is often called "Museum Heist" (because you Rob/Wall/Art).
Season 2 saw a lot of OT 3 involving Dick Grayson (especially in the tie-in comic), such as Barbara/Nightwing/Zatanna or Rocket/Nightwing/Zatanna. It helps that the series makes clear that is Dick not only still friends with all of his exes, but his exes are still friends with each other.
