I would like to see KH3 on the PS3 with being able to play it on the PS Vita. I would somewhat say PS4, too, but I'm not too sure about that. Yeah, the system would have a lot more power to it and would allow the game to be much better (provided the additional work is put in.) However,... Depending on how long it takes for the PS4 to come out and how long it takes for KH3 to be ready for release, I really don't know. In best interest, I think that the PS4 should be out for some time first if KH3 would be released on it. I know I wouldn't want to have to pay a ton of money to get a new system... I would do it for Kingdom Hearts, but it would be really frustrating and probably more than I could afford for a while. I imagine a lot of people wouldn't want to dish out a lot of money just to finally play KH3. If the system is out for a while first, at least that gives time for more people to get the system and for the price to eventually go down. Depending on how long it'll take for the PS4 to be completed, you'd be waiting that time plus extra (according to what I said) before you'd finally have KH3, though. :/ (I do realize the game may take a long time to be released regardless.)
Anyway, if anything, I'd say if it is on the PS4, for it to also be on the PS3 (still with the ability to play on the PS Vita, please :3) in best interest of the fans and the series.
I could comment on the other consoles, but meh... Not right now, at least. I don't feel like getting into all of that. >_>