Somebody has to take control of the crew during the game. As described, the two of you with experience should have gotten your heads together early and agreed to carry the new guy's water. But after the game, talk to the coach and tell him you share his concerns and let him know it will be addressed. And then address it. Make sure your association doesn't drop a guy in a varsity setting never having done 3 man before, and let your assigner know this newbie is not ready for primetime.
Its either that, or have the newb fake an injury so you can close out the second half 2-man!
