I think its because the third Keyblade warrior is Mickey. He is the only one who actually had a mastery test under Yensid, thus Riku and Sora will also have a Mastery test under Yensid, making them Master Yensid's pupils. Kairi was able to weild a keyblad because of the light in her heart, but remember Riku was the one who made it for her in KH2...so really Kairi is not a keyblade warrior....but it would be nice if like at the shocking ending that they reveal that not only is hse a Princess of Heart, she is also a Keyblade warrior. Or maybe the ending will show a brand new enemy! Or maybe Vanitas will show up to fight them both...or during the Mastery Exam, they encounter a soldier that looks an awful lot like Terra (or Aqua). If it looks like Terra, then all of Terra alter Egos will be in the game. (his heartless and nobody) If its Aqua, then she will probably run off after fighting them, leaving them wondering who it was and why. Or maybe there is a secret ending leading to KH3...
