[phish]

Even tho it was not pre-planned, this IS collusion. You bet knowing that you would not win the pot, but is rather providing the opportunity for another player to win it. THAT is collusion.
Fair poker is each player acting solely in his own self-interest. Dave did the correct thing, raising to drive out the winning hand so he can win. But you had bet that river knowing it would only provide Dave the opportunity to do this, without real expectation that your bet alone would win the pot for YOU.
That third guy was right to go bonkers. You cheated him.