It certainly won't be a Wii exclusive, but I dunno if the Wii can be discounted. Square-Enix clearly uses a PC engine in-house, and it would be an in-house team developing KH3, most likely. It would likely be based on the same engine FF13 uses, so that means that its practically on the PS360 already.
However, the Wii's fanbase... and Disney... man you know that's gonna go a long way.
I'm wagering on a rare triple-platform release, and that the PS360 versions are higher resolution (with more detailed textures), but not really on par with Final Fantasy XIII, due to the Wii's... umm... (I'm trying to say this without upsetting the fanboys) lesser stature?
Square-Enix and Disney have the muscle to release on whatever platforms they choose... and thus they will. They are in it for the money, after all. Pre-existing PS360 engine == low dev costs == money. Wii numbers + younger fanbase == money.
It'll hit all three.