loads of good commentary in this thread an most has been said so I'll make this brief: another factor that needs to be considered when wondering to go with the third wheel is whether or not the house PA can handle it! It is laughable to see a booth set up with three only to hear a crap PA, usually improperly eq'ed, between the needle and one's ears

so even if the three decks are in place, you won't be any less a man (or woman!) if you opt out on the side of quality when the PA can't do its job.