Crysis 3 is currently being developed. The release date is expected to be April 2013... unless there are delays.
You don't need to play Crysis or Crysis: Warhead since the story has been changed enough that many fans were confused why it was going to take place a city. Other than the fact that consoles would have a very hard time rendering jungle environments. Crysis and Crysis: Warhead are both better than Crysis 2 and more demanding on your system. Crysis 2 is too much of a console FPS game; meaning you only have one path to take to get to your objective.
There should be no difference in the game whether you buy a digital download from whatever site or the actual DVD.