How do you revive in sas zombie assault 3

It depends on which mode you are on. On Singleplayer you can't respawn again (which you lose.) But on Multiplayer you can respawn even if your the lowest rank. When you hit rank 27 you can respawn FASTER. So the answer is you can revive in Multiplayer but not singleplayer.

BTW: If everyone dies in Multiplayer at the same time then you lose.