The two sidekicks will usually be opposites in other ways, too — one will be a straight-A student, one will be a jock; or one will be rich, one will be poor; or one will be black, one will be white (or Asian). To that end, they may double as a Power Trio.
Romance will be explored. A Last Minute Hookup could almost always be expected between The Hero and a sidekick, until J. K. Rowling popularized the sidekick romance (although Lucas did it first with Han and Leia). One should hope that it won't turn into a Love Triangle.
The two sidekicks are almost always the Secret Keepers in a Wake Up, Go to School, Save the World setting. This is an attractive setup for many writers, because it gives the main character a person of each gender to interact with easily.
Most prevalent within American shows directed at children and tweens, where writers conspicuously try to keep their cast as limited as possible. Anime tends to add a fourth member, closer to one of the sidekicks than The Hero. They might also get a team expansion later, but the trio will always be front and centre and be treated like the "real" heroes.
Quite often the With a Friend and a Stranger dynamic will be present.
Is often Super Trope to Two Guys and a Girl, Two Girls and a Guy. Compare Token Trio. Contrast with the all-female Sister Trope, Beauty, Brains and Brawn.
Terrible Trio is the villainous equivalent: The leader will be more bossy, and the sidekicks dumber and interchangeable.
Not to be confused with the comedy film. Or that one Disney film barely anybody remembers.
Eyeshield 21: Sena, Monta, and Suzuna. The hero, his best friend, and his love interest, with the latter two serving as secret keepers for part of the series.
Also Akaba, Kotaro, and Juri.
Air: During Minagi's arc, there's Yukito, Minagi and Michiru.
From Ikki Tousen: Teifu (Cheng Pu), Ryoumou (Lu Meng) and Saji (Zuo Ci) or better said, Ouin Shishi (Wang Yun) from Nanyou. But their friendship is horribly crashed when Teifu becomes an Ill Boy.
All the Rookie Nine trios get this; as well as the older group (Neji, Lee, and Tenten).
Ranma ½: Ranma (Hero); Ryoga (Best friend/Rival); Akane (love interest of both). Also, Akane loves Ranma but fights with him constantly; she is not in love with Ryoga and only sees him as her best male friend, and never fights with him. Ryoga loves Akane and Ryoga (until he moves on) and Ranma border on Vitriolic Best Buds. And that is the simplest, least psychotic romantic situation in all of the Ranmaverse.
Both Ryoga and Akane are heavily implied to be Ranma's best friends, completing the essence of this trope even more. It kind of makes sense seeing as how they're the ones he talks to the most about his problems or thoughts. At least it explains why fans ship Ranma with Ryoga and Akane so much.
Higurashi no Naku Koro ni: Keiichi, Rena, and Mion are looked at as a trio. Satoko, Rika and Hanyuu may count as well.
Though Soul, Tsubaki, Liz and Patti are always there also so I'm not sure it counts. If it does than Ox (with Havar), Kilik (with Fire and Thunder) and Kim (with Jaqueline) would be this as well.
To elaborate: Mikado is the founder of the Dollars, Kida is the shogun of the Yellow Scarves, and Anri is the original Saika and the leader of Saika's Children. This makes the trio the leaders of the three rival factions in Ikebukuro. Suffice to say, It Got Worse.
Shigure from Fruits Basket said that himself, Hatori and Ayame were this during their school years.
Ouran High School Host Club has Tamaki and the twins, Hikaru and Kaoru. Or Tamaki, Kyouya, and the twins. Or Tamaki, Haruhi, and the twins... there are quite a few combinations, but the twins are closest to Tamaki and Haruhi (another reason why they tease and harass them, as opposed to the other club members).
Sailor Moon during the first half of the first season, with Usagi, Ami, and Rei, before Makoto awoke as a Senshi and Minako revealed herself as Sailor Venus.
Arguably, Luffy, Zoro and Nami from One Piece. At the start of the series, they are surely this, being the only members that make up the Straw Hat crew, and even in the later chapters they have shades of this - Luffy is the absolute protagonist, and even though Your Mileage May Vary on this, Zoro stills seems to be the most important character in the series after Luffy (being his Number Two and all) and Nami is probably also the most important female character on the show. Their romance hasn't been explored in the series, though, but it is heavily discussed in the fandom, and some fans will insist that there are plenty of hints.
Except those fans are wrong, because Eiichiro Oda himself has stated that there will never be any romance among the main characters in One Piece, because he thinks it distracts readers from the main story. The only member of the crew who has a love interest is Usopp, who has Kaya waiting for him back at his home island. And even then, Kaya's not officially stated to be his girlfriend, but every time she's mentioned it's heavily implied that she's supposed to be.
Trinity: Superman, Batman and Wonder Woman (lampshaded in the first series arc, during which Wonder Woman even mentions her brief flirtations with both Superman — the archetypal ideal husband — and Batman — the archetypal intriguing bad boy.)
The Mystery Kids: Holly (Ego - the most normal one, uses reasoning to solve mysteries), Miranda (Id - prides herself on her ability to crack terrible jokes, most likely to be inspired to a solution), Peter (Superego - collects car registration numbers, loves technology and most likely to use a gadget to find a solution).
Power Rangers S.P.D. does a fake-out with this trope: The ranger team starts out as Sky, Bridge and Syd, who fit the trope closely, but after Jack and "Z" arrive, they get rounded out to a Five-Man Band by the end of the first episode.
Merlin has one of these, despite the setting. Arthur, Merlin and Guinevere don't attend high school, but they make up the aspects of the trope perfectly.
Greek has Evan, Cappie and Casey before the series began but it ended in it's logical conclusion, later they try several times to come back to this but they fail.
Professor Layton: Layton, Luke and Flora. (In the fourth game, which is a prequel to the three currently available in the United States, it's Layton, Luke, and Remi.)
Persona 3: The main character, Junpei and Yukari, at least early in the game before the whole team pulls together into a group of genuine True Companions. Mitsuru, Akihiko, and Shinjiro were Three Amigos in the early days of SEES, but fractured prior to the events of the game.
Persona 4: Main character, Yosuke, and Chie at first.
The lowerclassmen (Rise, Kanji and Naoto) may fit too.
Elite Beat Agents: Agent J, Agent Morris and Agent Derek. On other difficulties, J is replaced by Agents Spin or Chieftain, and on the hardest difficulty, they are all replaced by cheerleaders (the "Elite Beat Divas").
Chrono Trigger: Chrono, Lucca and Marle of the 2 girls and a guy variant.
Final Fantasy VIII:: Laguna, Kiros, and Ward; Rinoa, Zone, and Watts; Seifer, Raijin, and Fujin for a villanous version.
Pokémon games after Generation II: protagonist, rival, and love interest/friend.
Metal Gear Solid: Guns of the Patriots gives us Snake, Otacon, and Sunny.
Planescape: Torment: Although you can recruit other characters, the core trio are The Nameless One, Morte and Annah.
Almost every video gaming webcomic ever seems to revolve around two male gamers and one female, who is usually the girlfriend of whichever male lead is the hero. The typical character dynamic seems to be 'wacky' (read: obnoxious Jerkass) guy [The Hero], straight man [second guy] and girlfriend [the woman]. Oddly, the dean of video game webcomics, Penny Arcade, is actually the exception (no girlfriend), so Ctrl+Alt+Del is probably the Trope Maker here.
El Goonish Shive started out this way, with Elliot, Tedd, and Sarah, before the cast expanded.
Slightly Damned has Kieri (an angel), Buwaro (a demon) and Rhea (arguably the main character). Buwaro and Kieri are dancing around a romance.
Woods For The Trees has Jess (Male Border Collie), Reuben (Male Fennec Fox) and Isabel (Female Red Fox).
[1] has Princess Raeka, Jacquline and Lyonel. A girl and a boy about the same age who recently have been put in a situation of romantic tension and the little girl to play foil to the main character.
In Rescuing Dara Chivai is the hero and a girl, Dara is the female friend, and Zilch is the male friend. No romance yet, though.
Avatar The Last Airbender Season 1: Aang, Sokka and Katara. Slightly unusually in that Sokka and Katara are siblings (and therefore have really similar experiences). Rather typical still in that they have very different personalities and capabilities.
The sequel series The Last Airbender: Legend Of Korra is slightly averted, as The Hero, Korra, is female and her friends, Mako and Bolin, are male. But like Katara and Sokka, Mako and Bolin are siblings.
Kim Possible: Kim, Ron and Monique. Although the trio dynamic is only really in play during the High School part of Wake Up, Go to School, Save the World, they have occasionally been involved in fights as a team.
But for the "same gender friend", the same dynamic works on the Save The World part, with Kim, Ron and Wade.
The Wonder Pets has Linny, a gender-ambiguousfemale guinea pig that is supposed to be equivalent to a 5-year-old human child; a male turtle named Tuck, equivalent to a 4-year-old, and a female duck, equivalent to a 3-year-old, with a speechproblem.
Jimmy Neutron, Sheen, and Carl may be all boys, but Sheen is dumb while Carl is smart, and Carl even called the trio the Three Amigops (a typo that he stuck with).
The Simpsons: Bart Simpson, Milhouse Van Houten, and Samantha Stankey in one episode called "Bart's Friends Falls in Love". Lisa would fill Samantha's role on an occasional basis.
