I had standard matching activated and was matched with another player. 3-player is unchecked by default. We played a row of games and in between the games a new player appeared at our table. Surprised why he came to our table when he wasn't a friend to either of us, he told us that he was matched by the tool to our table. Neither me nor my opponent had asked for a 3-player game, so the matching tool shouldn't have sent the other player to our table.