The Nets lead the best-of-seven series 3-1 and were in control at the TD Garden early, taking a 13-point lead into half-time.
Harden added 23 for the Nets – with the Big Three accounting for 104 points – while Jayson Tatum lead Boston’s scoring with 40.
The Atlanta Hawks overcame the New York Knicks 113-96 to lead their series 3-1.
New York’s Julius Randle was limited to 23 points, while Trae Young scored 27 for Atlanta and Danilo Gallinari added 21 from the bench.
The Los Angeles Clippers led with 9:30 left in the first quarter and did not look back to level their series with the Dallas Mavericks 2-2 with a 106-81 victory.
Kawhi Leonard top scored with 29, while Paul George added 20 in the win in Texas.
Elsewhere, the Phoenix Suns drew their series level with the Los Angeles Lakers at 2-2 with a 100-92 victory.
All five Suns starters finished with double figures while only one Laker managed the same feat, LeBron James scoring 25.