Ambassador - Edge case @ end of game with 1 victory card remaining - you could Ambassador it to allow your friend (again on your left) to gain it.
In a game with Amb and Lighthouse, you could choose whether to Ambassador good cards or bad cards depending on whether your friend or your opponent have lighthouses out.
End-game - PPR tricks. If your friend is ahead, buy the penultimate province, leaving your opponent to buy a Duchy and forced not to buy a Province. If your friend is behind, don't buy the penultimate province.
Or, build up an engine that does nothing but generate lots of buys. A hamlet-market engine that, by the end of the game, can generate 10 buys and empty the curses in one turn, letting you end the game whenever your friend is ahead. Similar in spirit to the 3-pile rush you suggested, I suppose.
Pull a KC-Goons-Masq trick! In 3p, this destroys the deck of only the player to your left.
Denying cards. For example, if my friend goes for Big Money, I could buy up all the villages to ensure that an engine can't get off the ground.