Uh well, forgot about that, copied a part out of my TD. Well it would be easier to trigger, if you tell us what kind of map you are working at. We need to now how dynamic this trigger has to be. Maybe there is the posibility of this setup:
Player 1 allies Player 2 and 3
Player 3 allies only Player 1
More Information are required before i'll post another trigger.
Well and sorry for that wrong Trigger.