I think X-3 means the Keyblade Wars Video.
Heh, couldn't find the proper one, too many damn fan made re-mixs.
The Keyblade Wars video was put in the Kingdom Hearts 2 games as a what could happen in the future of KH. (Apparently he originally done it in the first one. Due to the demand he carried on with the series(Don't ask me who said it. I can't remember it was in an official Playstation Magazine))