Well, I saw some of those images in the "secret ending" for KH2 (The field of keyblades, which are the sword-y looking things) so at least some of it is authentic. And I know that they recently released a "remix" of KH2 in Japan, which included a version of the Game Boy game as well, so this may have been included in there as well. And Mickey, well, avoiding spoilers, I'll just say that's probably legit as well. So, I'd guess that this is likely a real trailer. I don't think KH3 has been announced yet, but I don't think anyone would be surprised.
Right now, as far as I know, the only KH-releated hoax is EGM's announcement that a Wii version of the game will be coming out with Mario and the rest of the Nintendo characters included in the game.