Who knows? SE likes to makes spinoffs, so maybe KH will get a Wii version while the main KH franchise remains on a Sony platform.
Depends on how well ps3 does in Japan. As it is they're selling about 1/3 the rate Wii is so if that continues, I imagine it will be pretty hard for jrpg devs to ignore that userbase.
My prediction is if PS3 does get KH3 it will be a port from Wii. Jrpg's will be mostly on Wii this gen and ported with a bit of polish in certain cases on ps3 (and some to xb360 too just out of ease of development).
All dependant on how well ps3 does in Japan of course. If they turn it around we could be looking at the reverse but Japan seems to bet heavy on one horse early and ride it to the finish line. We'll see.