The star pairing of guard Jaylen Brown and forward Jayson Tatum led the way for Boston who were without centre Kristaps Porzingis due to a lower leg injury sustained in their game two win.
Brown had 30 points, eight rebounds and eight assists, while Tatum had 31 points, six rebounds and five assists. For the Mavericks, Kyrie Irving had 35 points, while Luka Doncic scored 27 before he fouled out in the fourth quarter.
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The Celtics held a 21-point lead early in the fourth quarter before Dallas staged a comeback, bringing the deficit to just one point as they only allowed Boston to score two points during that period.
But as Doncic fouled out, the Celtics held on to secure the win in the clutch moments.
No team has come back from a 3-0 deficit in the NBA Finals and Boston are just one win from raising their 18th championship banner – breaking a tie with their rivals the Los Angeles Lakers for the most titles.
Both teams paid tribute with a moment of silence for former Lakers player, executive and NBA hall of famer Jerry West who died on Wednesday aged 86, the first player to be recognised as the NBA Finals most valuable player.
The Celtics will have a chance to complete the sweep on Friday when they collide with the Mavericks in Dallas for Game 4.
“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 totalling 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.
Like Tatum, Irving made up for a lacklustre showing in each of the first two games of the series, and he paced the Mavericks with a game-high 35 points. 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 three-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. PJ Washington and Irving both missed three-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.”
