Very easy to unlock. If you start the first Pacific circuit game on rookie, you'll likely be able to shut out your opponent and unlock this after your first game.
The easiest games for me to get 3 stars was on Rookie difficulty with the goal limit set to 3.
You'll learn quickly that you get points (which determine your stars) by the number of shots you have, and the number of goals you score. Conversely you lose points by the number of goals your team gives up, and the shots your opponent has. You also get a bonus for a shutout. Generally a normal game winning 3-0 on rookie difficulty would give me plenty of points for 3 stars.