In Games 1 and 2, Spurs forward Kawhi Leonard was passive. Whether it was foul trouble, adjustment to the speed of the Heat or just mental difficulty, he was held in check in large part. But in Game 3, Leonard was absolutely venomous, striking on defense and attacking on offense. The result was a 21 point first-half lead for San Antonio. Leonard finished the first half with 18 points, three rebounds and three assists.
It's key for Leonard to be aggressive. It forces LeBron James to work harder on defense, and Leonard's strength allows the Spurs to attack the rim, which opens up their perimeter offense (which is just as key). It's a huge part of the game. And it's not just obvious plays like his shooting. Ray Allen tried to save the ball off a steal late in the first half. Leonard zipped in front of LeBron James and saved the possession, which lead to a Diaw 3-pointer. It's those plays that can make a huge difference.
The Spurs shot unbelievably well. But Leonard made the difference by setting the tone. The Spurs need him to be part of the Big Four. In Game 3, he's shown up for it.