The Celtics pulled away after a 20-point lead shrunk to one early in the third quarter and put themselves in position to tie a series that was in danger of slipping away from them. Game 4 is Sunday in Chicago.
Dwyane Wade scored 18 for Chicago.
Thomas, grieving the death of his sister in a car accident last week, rejoined the team after spending time with his family in Tacoma, Wash. He scored eight points in the third quarter, helping the Celtics regain control.
Jae Crowder finished with 16 points.
The Celtics' lead was down to 44-43 early in the third when Thomas hit Horford for an alley-oop dunk and nailed back-to-back three-pointers. Boston maintained a comfortable edge after that.
The Celtics led by 18 after sinking seven of 11 three-pointers in the first quarter and were up 37-17 early in the second before the Bulls jumped back into it.
They had the crowd rocking in the closing minute of the half. Nikola Mirotic nailed a three-pointer, cutting it to 44-41, and pointed toward the sky. Wade then blocked Bradley's three-point shot, causing a shot-clock violation with 4.7 seconds left, before Butler missed a desperation three-pointer for Chicago as time expired.
Russell Westbrook scored 32 points while recording another triple-double, and the Oklahoma City Thunder held off the
Westbrook also had 13 rebounds and 11 assists. It was his second straight triple-double and the seventh playoff triple-double of his career.
The Thunder blew a double-digit lead in the second half of Game 2, and barely hung on this time. They host Game 4 on Sunday.