im guessing you are talking about the birth by sleep video, which is the secret ending for kingdom hearts: final mix+. it is actually part 2 of the trailer for the psp game titled birth by sleep. part 1 was the secret ending in kh2. birth by sleep will be using the same engine as final fantasy: crisis core. the ds game is called 358/2 days. hopefully they will come out sometime next year. there is also a mobile phone game called coded. i have no info on kh3. none has been released yet, except some possible clues from kh2: final mix+ in the secret areas of the game. keep up to date at
http://www.kh2.co.uk/ and
http://www.kingdomhearts3.net/