LP Loss?

Well LP gains/losses depend on relative mmr, so when you're the highest mmr you're always going to be expected to win, and so the LP losses can be pretty harsh.

Plus I don't know how populated 3v3 land is, but it doesn't seem a stretch that theres few enough groups that you might get matched against relatively low mmr'd people for extra painful LP losses (although it 'should' be a free win because of the mmr difference).