On the BGG thread someone came up with a way to get a turn 2 Province in a 3-player game. I can do one better, although admittedly a lot more far-fetched.
Turn 2 Platinum:
TURN 1:
Player 1 buys a Noble Brigand.
Player 2 buys a Council Room and shuffles.
Player 3 buys a Masquerade and shuffles.
TURN 2:
Player 1 does whatever.
Player 2 plays his Council Room, which has magically appeared in his hand due to Player 1's Noble Brigand. He now has at least 6 Coppers in his hand, so he buys a Gold and shuffles.
Player 3's hand, having been aided by Player 2's Council Room, is a Masquerade, an Estate and 4 Coppers. He plays the Masquerade, draws two more Coppers, and passes his Estate to Player 1. Player 2 has a brain aneurysm and passes his Gold to Player 3. Player 3 now has in hand a Gold and 6 Coppers, and happily buys a Platinum.