Two guys + one girl is all but standard structure, and the possibilities with it are all but endless. Like with Aqua - she has to look out for the two best friends. Xion - has the two look out for her.
Kairi breaks it though because she's fighting with Riku over Sora :\
Right, we have friends who take care of others, friends that are supported by others, and friends that do everything they can to destroy the friendships.