Have a trigger with condition of:
!alive tank1 && !alive tank2 && !alive tank3
Set it's Type to Switch and synch the trigger to the HOLD waypoint. When all three tanks are dead it'll release the hold waypoint. You can't change it from HOLD to MOVE though, you'd have the MOVE already plotted.
Also, this is another example of why waypoints for players are silly because a HOLD waypoint won't do anything for a player and I'm fairly certain any AI with the player will follow the player instead of the waypoint.