Garnett has the most impressive resume and the best stats. He leads the Celtics in scoring and rebounding, and also in shots per game (13.9). However, his shot total is ever so slightly padded because he spends more time near the basket and subsequently corrals more offensive rebounds (1.9, compared with 1.0 for Allen, and 0.7 for Pierce) and therefore more put backs.
The biggest problem with going to Garnett on a steady basis in critical moments is that his penchant for launching fadeaway jumpers rarely gets him to the stripe (4.7 free throws per game). Nor is he a particularly adept passer (3.5 assists). Because the ball must come to him, he can also be more easily denied possession than either Allen or Pierce.
Moreover, there is the question of KG's reliability in the clutch. In fact, several veteran NBAers privately voice the expectation that Garnett will find a way to fail when a game is on the line anything from missing free throws to missing shots, from turning down easy shots in the paint to fumbling away key rebounds.
So there are several reasons why, if he had his druthers, Rivers would put the ball in Pierce's hands in the clutch. Pierce gets to the foul line more than Garnett. Pierce is a better passer and a better handler. Because he can go get the ball, defenses have trouble overplaying and denying Pierce possession. Defenses are also unlikely to double PP when he receives the ball in the middle of the court. Because of his superior mobility and the unpredictable variety of his moves, Pierce can always be depended on to generate something positive for himself or his teammates.
End-game matchups, foul situations, and the respective hand-temperatures of Allen, Garnett and Pierce are always to be considered when Rivers selects his shooter-of-choice in the waning moments of a tight ball game. Still, while Garnett is most likely the only constituent of the Celtics latest Big Three who's destined for the Hall of Fame, Pierce is usually the team's top gun whenever a sure shot is absolutely necessary.
The outcome is that for the Lakers to prevail, they certainly have to limit the effectiveness of Allen and Garnett, but they absolutely must contain Pierce.