In bot games, emptying the Curse pile will not count towards a 3-pile ending. I have multiple games now where I have three piles empty, including the curse pile, but the game continued several more turns until a fourth pile was out.
This hasn't yet been tested in rated games.
See game #96798563 on frankfurt.
That game you mentioned was a 6 player game. For 5 and more players, 4 piles have to be empty to cause the game to end (Province/Colony pile being empty is still also a game ending condition). I assume that your other games were also with 5 or more players.