Unless this was a Domination game, the other player who dropped was wrong.
2v3 is totally winnable if there are no AIs, especially on Crucible. I've played it on both sides; when I was on the 2 side I won both times, and when I was on the 3 side it was tough going before I got kicked by network problems. You just need to know the game mechanics and how to exploit the situation. Those have been some of the most fun matches I've played, and I hope Stardock leaves this capability in.
Now, if there are AIs (i.e., it's 2 + 1 AI vs 3 or worse, 1 + 1AI vs 3), you've got no chance.