I play almost exclusively 3-4 player games and, practically speaking, the kingmaking you describe - holding off buying province or three pile so player A can win instead of player B - never comes up. The common attitude is, if I'm not going to win, then I want to end the game as quickly as possible so I can play another round I could win. It becomes one of the unspoken rules, and you take it into account when you play (ie, if you still can win, you avoid making it easy for anyone to end the game before you can catch up.)
Sure, but your group having a consistent priority doesn't make it the "right" choice, and since it is still a choice, it's kingmaking. This topic has come up before as well. What if you're in a tournament setting where 2nd place is better than 3rd? What if you're a new group without established conventions?
Another common attitude among the general gaming population is to target the leader, which is the opposite of what you're describing (trying to end the game sooner helps whoever is in the lead).
As a general rule, I would consider it to be bad sportsmanship purposefully make make a move that guarantees you lose the game. Especially if that decision specifically helps one player over another.
The problem comes when a player is literally mathematically eliminated from the game; which can certainly happen in 3 player Dominion... player A has 6 Provinces, player B has 5 Provinces, and player C, who is really new to or really bad at Dominion, has none. For argument sake we could say that a bunch of Duchies are gone too; so that it's a literal elimination, but even with 12 Duchies available, it's reasonable to consider yourself eliminated even then; you know you aren't going to get every single Duchy plus some Estates before anyone gets a Province (or just buys some Duchies/Estates of their own). In these situations, there's a valid question that the player has to ask, which is "what is my goal now"? There seems to be 3 (legitimate) possible goals you can have:
1. Get the highest place possible; do the move that gets you second place.
2. Maximize your own score; no matter how it affects the ranking.
3. Minimize the difference in score between you and the winner; come as close as possible to winning.
I don't think I can fault anyone for choosing any of those 3. But it needs to be a situation where actual victory is not an option.