NHL Threes Mode is back with Threes Circuit 2.0. The Circuit King achievement is unlocked by winning the last game in the Legends Circuit (Montreal Canadiens Alumni).
EA did us a favor this year and reduced the number of games down to a total of 64. You'll need to complete both the Western Conference Circuit and the Eastern Conference Circuit, earning 3 starts in each game to unlock the Legends Circuit. Earning 3 Stars in each game can easily be completed by playing on Rookie with a three goal match requirement or scoring at least 3 goals in a periods game.
This achievement can also be boosted by playing co-op with someone who has already unlocked the Legends Circuit and playing final game against the Montreal Canadiens Alumni. In doing so, you'll also unlock:
The Cooperative achievement in NHL 19 worth 71 pointsComplete and win an Online Co-op NHL® Threes Circuit Game.
*EDIT* : Thank you TheWrestlingGod for confirming that there is a requirement for earning 3 stars in each game.