Main character ensembles

Although many stories have just one main character, many others have an ensemble of main characters, usually in sets of three to five. Although one character in these ensembles may be presented as more central to the story than the others, technically, all the characters in the ensemble could be considered main characters. For instance, a lot of classic superhero programs, especially on television, feature a team of five superheroes, often with one or two of that number being female. The children's musical comedy "Fraggle Rock" also has five main characters (three male and two female), while "The Office" and its permutations have four (three male and one female). If there are three main characters, they're often all of the same gender (e.g., Alvin and the Chipmunks), but an arrangement of two males and one female (as in the "Harry Potter" and "Twilight" series) also seems to be popular. Why are these particular configurations of main characters so common? Are there any stories that invert the usual gendered expectations about the makeup of the groups (two females to one male, for example)? Moreover, why does any story need more than one main character in the first place?