Odds are you've seen them. Perhaps as you read a description you will even recognize them. But do you understand what they are and how they work? I speak (er...write) of a certain trope or character dynamic dubbed (by yours truly) as the Heroic Trio.
First of the trio I call the Decider. In whatever essential conflict of the story, he or she will be the one to...well...decide. Another way to look at it is they have the ultimate responsibility in resolving the central conflict of the story. Be it to defeat the Dark Lord, or rescue the princess from the evil galactic empire, or solve the mystery of who is the werewolf threatening the entire town--the Decider has to make the central choices leading to success or failure.
Helping the Decider are two others--companions or co-workers or family members or something. What distinguishes these two are the roles they play.
One I call the Ace, because first and foremost this person is an expert. At what entire depends upon the story details. Sometimes they are a brilliant scholar or scientist, possibly the brightest student in school. Then again, maybe they alone know how to enter the secret lair of the evil wizard.
Finally there is the Supporter, the person who acts as an emotional anchor for the trio. In practice this often means a humorous person, something of a clown, but not always. A Supporter might just as easily be the equivalent of a butler or nanny or even a bartender -- someone who helps emotionally and physically.
Kirk has Spock and Bones. They also function as two sides of Kirk's personality, or at least the voice of each. An interesting aspect of that dynamic is that Kirk leans so heavily on Spock precisely because Kirk tends to be so intuitive rather than logical. In heroic trios this often happens, that the Decider becomes in some ways more associated with the one who is least like them. Fan fiction abounds with tales of Kirk and Spock, their famous friendship, at times positing a romantic/sexual aspect to it. The new Star Trek movie was almost completely built around that relationship.
Buffy has Willow and Xander. At one point in the series they were explicitly labeled the Hand, the Head and the Heart -- during a season that was explicitly about reforging the trio in the wake of graduating high school. Willow and Xander are extremely close friends, having known and leaned upon each other practically since infancy. Yet not until Buffy arrived did they begin to find focus in their lives. Again, this is common aspect of the heroic trio. The Decider becomes the focus, the point around which the other two orbit to some extent.
Frodo has Gollum and Sam, which helps illustrate another common trait. The Ace and the Supporter often seem to be polar opposites. In Lord of the Rings it is harder to imagine a more typical Hobbit than Samwise Gamgee, nor a less typical one than Smeagol/Gollum. One is obsessed by the Ring, addicted to it, both loving and hating it as he loves and hates himself. The other is among the tiny minority almost unaffected by the thing. Even their eating habits are (famously and comically) at opposite ends of the spectrum. One is ancient, the other young. One is fat, the other scrawny to the point of emaciation. One loves plants and gardens, while the other is most comfortable in underground caves.
Vlad from Young Dracula has Chloe and Robin, who highlight how the desires of the characters in a heroic trio are usually intertwined. Vlad is a vampire who wants to be human. Robin is a human who wants to be a vampire. Chloe is the clear-sighted one who clearly sees that hanging around a real vampire is very, very dangerous -- yet continues to do so (Robin is her brother after all, and Chloe does seem to genuinely like Vlad).
Harry Potter has Hermione and Ron. That trio was formed early on in the very first novel of the series and once continuing theme is how their relationship is stretched and bent, net never quite truly broken. It also illustrates something universal in the heroic trio--the Ace and the Supporter have a powerful emotional relationship. They never exist separately and are never indifferent to one another. In this series, they fall in love. Elsewhere they are siblings, rivals, hate one another, are deeply loyal in contentious colleagues, etc. No matter the details, when these two characters interact, some kinds of sparks fly. Unless of course the trio is dysfunctional for some reason...
Looked at in a certain way, Bill Compton on True Blood has Eric and Sookie (this is from the perspective of Bill's character of course--the series itself generally is from Sookie's POV). In The Dark Knight you can see that Bruce Wayne/Batman has Alfred and Rachel Dawes (the Butler in this case being the Ace). Dexter violates this trope in that he always seems to be trying form such a trio -- but they collapse, not least because in terms of the character dynamic he has to keep his Ace and Supporter apart. This fits into his particular storyline, that of a serial killer trying to turn into a normal human being (a quest that, if successful, would of course destroy Dexter completely -- talk about an inherent and unresolvable conflict!). On the opposite legal end of the spectrum is the intense and often-brilliant Kurt Wallander (played in English by Kenneth Branaugh on Masterpiece Mystery), much of whose internal conflicts can be seen from a near-total lack of anyone who can function as part of a trio with him. He is alone, and that fact tortures him.
Bones has Booth and Angela. House has Wilson and his team (being a bit dysfunctional, his Ace is a group whose membership keeps changing). Like any such trope, the Heroic Trio is a tool -- something to help and clarify things for the writer. Its dynamic can be useful. Be warned however of using it as a crutch or a formula for success. It is and remains only a tool.
Unclicking my soapbox icon. For now.