When is dragonball raging blast 3 coming out

Never. Thus far the developers haven't picked back up on an old IP for the DBZ games (they make a few Budokai games, then a few Budokai Tenkaichi games, then Raging Blast, and now they're on Ultimate Tenkaichi).

A DBZ game has come out every year since Budokai 1, so you'll see new games. Whether they're anything like Raging Blast, you'll have to wait and see.