At the moment there is no Kingdom Hearts 3 nor have I heard any official word on it. There is Kingdom Hearts: Birth by Sleep which was recently released on the psp which may be what you are thinking about. But I have heard nothing about it getting moved to the ps2 so I can't help you there either.
Now if they do make a Kingdom Hearts 3 which I am sure they eventually will as it is a huge franchise, it would seem they would use the ps3 instead as they are doing with FFXIII right now.