The Celtics went 20 for 40 from three-point range after going just 25 for 100 in their two losses in Boston, when they blew 20-point leads in the second half of both games.
The defending NBA champions went ahead by 31 in this one and there was never anything resembling a comeback for the Knicks.
“You’ve got to beat us four times. That’s what it comes down to. Not twice, not once, not three,” Celtics forward Jaylen Brown said.
“You’ve got to win four games, so there’s a lot of basketball to be played.”
Tatum, an All-Star who shot just 12 for 42 overall in Boston, and Pritchard, the NBA Sixth Man of the Year, each made five three-pointers.
Brown scored 19 points and Derrick White had 17 for the Celtics, who will try to tie the series in Game 4 before returning to Boston for Game 5.
Jalen Brunson scored 27 points and Karl-Anthony Towns had 21 points and 15 rebounds for the Knicks, who blew a 2-0 lead in this round last year and are still seeking their first conference finals appearance since 2000.
In the Western Conference semifinals, Anthony Edwards hit a baseline three-pointer with 1:19 remaining and scored 36 points, Julius Randle had a triple-double of 24 points, 12 assists and 10 rebounds, and the Minnesota Timberwolves beat the Golden State Warriors 102-97 for a 2-1 lead.
Jimmy Butler had 33 points, seven assists and seven rebounds and Jonathan Kuminga scored 30 off the bench, but the Warriors still lacked the kind of rhythm they have with Stephen Curry on the floor.
Kuminga shot 11 for 18 as the Warriors again mixed and matched while playing without Curry as he nurses a strained left hamstring that he injured early in Game 1.
The Warriors struggled to establish their long-range game. They didn’t make a shot from deep in the first half, but led 42-40 anyway.
Golden State missed its first five tries from beyond the arc before Buddy Hield connected 35 seconds into the third quarter.
They made six of 11 three-pointers in the third quarter and finished 10 for 23 from deep.
The Warriors came into Game 3 against Minnesota with a playoff-leading 135 three-point shots. (AP)
