I can see the PS3, Wii, DS and PSP all getting some kind of Kingdom Hearts game, but just one will get the big one: Kingdom Hearts 3. I think we can eliminate both handhelds, just because of the nature of the series it would be harder to justify that move as compared to something like Dragon Quest IX which is going to the DS.
I think it can go either way between the PS3 and Wii. In the PS3's favor is pushing technology, and the fact they can ride the momentum the system gets with the hardcore RPG fans after FFXIII comes out. In the Wiis favor is it's success, and potentially some neat ways the controls could be implement, though that theoretically can be said about any number of major series that aren't coming to the Wii. My gut feeling, the PS3 gets the main game, spinoffs for the Wii, DS and PSP.