DALLAS >> The Boston Celtics, one win away from sweeping the NBA Finals, insist they won’t get overconfident.

They only need to look back at the fourth quarter of Game 3 to see how dangerous the Dallas Mavericks can be.

Jayson Tatum poured in 31 points and Jaylen Brown supplied 30 as the Celtics held off a late rally by the Mavericks for a 106-99 victory on Wednesday.

By taking a commanding 3-0 lead in the best-of-seven series, Boston pulled within one victory of the 18th title in franchise history. The Celtics will have a chance to complete the sweep on Friday in Game 4 in Dallas.

“We are not relaxing or anything like that,” Tatum said. “You know, not even focusing on winning or Friday or whatever. However long it takes, that’s our motto. However long it takes, that’s what it’s going to take. Nobody is not trying to relax at all.”

Tatum bounced back on Wednesday after totaling just 34 points in Games 1 and 2. Brown added eight rebounds and eight assists for Boston, which also got 16 points from Derrick White. Big man Kristaps Porzingis (lower leg) didn’t play after getting injured in the third quarter of Game 2.

Like Tatum, Kyrie Irving made up for a lackluster showing in each of the first two games of the series, and he paced the Mavericks with a game-high 35 points. Luka Doncic went for 27 points, six boards and six assists, but he dealt a critical blow to Dallas’ comeback hopes by fouling out with 4:12 left in the game.

The Mavericks challenged the foul on Doncic, but the call stood, forcing him from the game. Doncic picked up four of his six fouls in the fourth quarter.

“We couldn’t play physical,” he said. “I don’t want to say nothing. You know, six fouls in the NBA Finals, basically I’m like this (motions with palms out). C’mon, man. Better than that.”

It looked as if the game were over when White drilled a 3-pointer with 11:07 left in the fourth quarter to put the Celtics up 91-70.

But Dallas then took over, rattling off 28 of the next 37 points to get within 100-98 on a Dereck Lively II dunk with 1:20 to go.

“You were going to expect a run from them because they had a 19-point (third) quarter,” Celtics coach Joe Mazzulla said. “And just by the type of shots they take alone, you knew they were going to make some kind of run. Some of those came at the expense of our offense.

“I thought we missed a couple reads that changed coverages and they changed matchups on like three or four straight possessions and we missed the read to get the right shot.”

Brown made sure Boston never let the Mavericks draw even, though, as his mid-range jumper at the top of the key increased the lead to four points with 1:01 remaining. P.J. Washington and Irving both missed 3-point attempts down the stretch before two free throws from White and two more from Tatum put the finishing touches on the victory.

Doncic’s layup had the Mavericks within 71-65 with 5:11 left in the third quarter. The Celtics answered with a 7-0 spurt and later took an 85-70 lead into the fourth.

After Dallas led by as many as 13 in the opening 12 minutes, neither team led by more than four as the lead changed six times in the second quarter. The first half ended with the Mavericks clinging to a 51-50 edge behind 20 points from Irving.

Boston outshot Dallas 46.3 percent to 44.2 percent for the game.

The Celtics also improved to 7-0 on the road in the playoffs while running their overall winning streak to 10 games.

The Mavericks will need a historic effort to overcome their Finals deficit. Teams leading NBA series 3-0 are 156-0.

“It’s not over till it’s over,” Doncic said. “We just got to believe. Like I always say, it’s first to four. We’re going to stay together. We lose together, we win together.”