DALLAS — Warm up the duck boats.

The Celtics have been the class of the NBA all season long. Wednesday night in Dallas they weathered a heavy storm in the first quarter as well as the likely loss of Kristaps Porzingis for the forseeable future.

But a dominating third quarter in which the Celtics outscored Dallas 35-19, followed by the first six points of the fourth to stake a 21-point lead, fueled a gritty 106-99 win and a 3-0 series lead in the NBA Finals.

Queue the ad nauseum reminder that no team in NBA history has erased an 0-3 deficit. This is over.

The Celtics proved their mettle all night. Dallas rallied back to cut it to 93-90 with 5:42 left, but kept Dallas from completing the comeback. When Luka Doncic fouled out on a close call guarding Jaylen Brown on a drive with 4:12 left. Kyrie Irving (35 points) got it to 93-92, but Boston wasn’t about to spit the bit now.

It doesn’t look like it will matter that Porzingis is out. The Celtics are that good.

After all, they rolled through his previous playoff injury by winning all but one game. They’ve gone 31-5 now in games he missed in 2023-24.

Wednesday night his absence was felt early. Instead of Porzingis off the bench to relieve Al Horford, halfway through the first period, coach Joe Mazzulla turned to … Xavier Tillman?

The Celtics outscored Dallas by 25 points during his minutes on the floor in games 1 and 2. But they were even without him. And the always game Horford desperately, at age 38, wants a championship as his career nears the end.

But the key stat is that: 38. And Horford isn’t really the guy you want chasing Dallas’ young bigs.

The evidence was all over the American Airlines Center floor in Game 3. Porzingis created an unsolvable matchup problem for Dallas’ high-flying but offensively challenged bigs. After pretty much walling off the paint in Games 1 and 2, Dereck Lively II and Daniel Gafford got to the rack for four dunks in the first half Wednesday.

A day after announcing that Porzingis might miss time with a “rare” leg injury — leg injuries have hampered his career from New York to Dallas and now to Boston (he escaped it mostly in Washington, but who knew?) — a few hours before tipoff Wednesday the club ruled him out.

“He didn’t look right,” coach Joe Mazzulla said 90 minutes before Game 3.

Mazzulla has options, but like other coaches taking teams this far he’s shortened up his rotation. An extended absence by KP — and let’s face it, the description of his injury hints that he’s done for the year — will have a trickle-down effect.

“Our depth just looks different,” Mazzulla said.

Horford was a rock, giving the Celtics 37 tough minutes they needed.

Still, Boston had a lot go right early. The defense handled itself creditably, despite the usual showing from Doncic (17 first-half points) and the long-dreaded Kyrie Irving breakout. Irving hit his first four three-pointers of the series in the first half, notching 20 points.

But the Celtics stifled Dallas’ unproven wings (2-9 at halftime) and Jayson Tatum shook off his shooting slump to put up 20 points in the first half and keep the game close. Tatum was on the floor for 22 minutes in the first half.

Boston needed his production — uncharacteristically, the normally reliable Jaylen Brown, Jrue Holiday and Derrick White (5-19 shooting in the first half, and only 1-10 from 3) were erratic.

Tatum cooled off big-time in the second half, shooting only 11-26 en route to a team high 31. No matter; his exceptional first half allowed Brown to take his turns in the second (30 points, 24 after halftime).

And now, within less than 48 hours, the Celtics could be flying back to Boston Friday night with that 18th banner.