I would say there is a chance it could be on any system. It really all depends on when development began. If KH3 development began more than 10 months ago, then it's most likely on the PS3. If it began sometime in 2007, it's most likely on the Wii.
However, there is always the chance that Sony or Microsoft bought the exclusive. This is a common practice for both companies so I wouldn't be surprised if that happened. Because of these factors, it could go about anywhere, and has an equal chance of going about anywhere. However, from a straight financial and logical standpoint, it would most likely be on the Wii.
As far as the spinoffs go, I'm guessing they will be spread out across 3 or so platforms. Off hand I would say that they'll be on the PSP, DS, and Wii. Of course, that could just be wishful thinking on my part since those are the 3 I own. I am positive though that at least one spinoff will be on one of those 3.
