What I would do is to put a point called "Player 2 attacks here" in player 3's base. Set this point as the target for player 2's waves. Then, add a trigger that moves your "P2 attacks here" point to player 1's base once player 3's base is destroyed.
There may be more elegant solutions, but that should work.
What I would do is to put a point called "Player 2 attacks here" in player 3's base. Set this point as the target for player 2's waves. Then, add a trigger that moves your "P2 attacks here" point to player 1's base once player 3's base is destroyed.
There may be more elegant solutions, but that should work.