So maybe Game 3 wasn’t a total loss, after all. The Dallas Mavericks, playing Game 4 from the start with the urgency and pace that fueled an ultimately futile 20-2 fourth-quarter run in Game 3, exorcised a lot of frustration and earned themselves a return trip to Boston with a resounding 122-84 win, reducing the Celtics’ best-of-seven series lead to 3-1.