Personally, I think KH3 will be released on PS3 AND PS4! It will serve as the conclusion of the Xehanort saga (and the end of KH on PS3) yet also as the bridge to a new saga (and the beginning of KH on PS4)
- Nomura said he wants to release a KH game every year.
--- 2013: KH HD 1.5
--- 2014: (presumably) KH HD 2.5
--- 2015: ???? --> My guess is KH3
Now my reasoning for the timeline:
-- Some people think KH X [chi] will be released in 2015 and KH3 in 2016. THAT"S SILLY!! KH X [chi] is already out in beta in Japan and should be out in beta in other countries around the respective releases of KH HD 1.5.
-- Some people think that BBS Vol. 2 will be released in 2015 and KH3 in 2016. Possible, yet Nomura already said they had no intentions of releasing bbs vol. 2 and also that KH3 should come next.
-- 2015 is the last "official" Sony supported year for the PS3 (as of right now). It would not surprise me if they allowed the system to go out with a bang with a game like KH3.
Of course, this is my own personal speculation. It could be right, it's probably wrong! Haha, it just seems reasonable to me.