Center Theis stood in the starting line -up, but only came to one point and three rebounds in ten minutes.
Under his possibilities, the last outstanding Jayson Tatum remained with just ten points.
The Celtics never led, but shortened a 62: 37 deficit to one point – 93:92, 2:40 Before the end by a threesome by Jaylen Brown, who scored 40 points.
In the heat that had to do without Jimmy Butler (knee injury) in the entire second half, Center Bam Adebayo convinced with 31 points and ten rebounds.
Game four of the series takes place again in Boston on Tuesday night.
Four victories are necessary in the Best-of-Seven series to move into the NBA finals.
In the second play-off semi-final between the Dallas Mavericks, Maxi Kleber team, and the Golden State Warriors is 0: 2.