So long as each player wants to do his best, and second place "counts" more than third, I think there is very little room for collusion or kingmaking in 3 player Dominion. Not zero, but very little.
No. For example: in a pile driving game, suppose I am last place, but can get the last Duchy and end the game. Unfortunately the Duchy is not enough for me to get to the second place. In addition, suppose the first two players are very close, and if I don't end the game the player who ends the game can win. Should I get that Duchy?
Or even in longer terms, suppose one player goes for a rush and the other player builds up. I fell behind both players, but should I get as many cards as possible to help the rush player, or just sit and watch?
Yes, lots of this situations technically comes from playing with the end game. But even discounting that, whether to play a militia, take a curse from the torturer, or get to a pile that is strongly competed can affect the other players differently. As long as their score are closer than yours to theirs, it can cause some form of king-making.
The only way to eliminate that is to require all the interaction from me to be strictly equal to the rest of players. Unfortunately it is usually not possible due to both the fact that players have to play in order, and that different strategies take the same interaction differently.
For the second point, simply put, say there is a paper-scissors-stone occasion. The other two player is playing paper and scissors. Do you play paper, scissors, or stone?