I read somewhere (probably ign or kotaku, can't remember which) that Square wouldn't comment on the PS4 and Xbox One versions of KHIII being released at the same time.
This leads me to believe that KHIII could be a timed exclusive for the PS4, or that release is so far out that Square didn't want to put anything to paper.
But wouldn't it be kind of cool if KHIII was a timed exclusive. The numbered entries have always been on Sony consoles, and Square has a much longer and lucrative history with them over Microsoft.
If it is a timed exclusive, then that is just more incentive for KH fans to go buy PS4s, because that's where you could play the game first. I would say it would be on PS4 for 3-6 months before coming to Xbox One.