The death knell may have sounded on the Nets’ disappointing season — killed by Kevin Durant’s surprising struggles and their porous defense.

The Nets lost Game 3 to the Celtics, 109-103, Saturday night before a disappointed crowd at Barclays Center and now trail in the series, three games to none. The preseason title favorites are now one loss away from elimination and staring right down the barrel of an embarrassing first-round exit.

“S–t every game is a must win from here on out,” Andre Drummond said beforehand. “We lost a tough one in Game 1 by a buzzer beater. In Game 2, we gave up a 17-point lead. So, we have the tools to win the game. We know what they’re capable of, we know what they’re going to throw at us, and it’s now just putting it forth into the rest of the series.”

No team in NBA history has ever come back from a 3-0 deficit. And nothing the Nets have shown through the first three games has engendered confidence they can be the first.

Bruce Brown scored a team-high 26 points, but Durant and Kyrie Irving (16 points) came up short for the Nets.

Durant, who has been held in check by Boston’s top-ranked defense, mustered just 16 points Saturday. For the first time in his career, he had been held under 40 percent shooting with six turnovers. Nets coach Steve Nash had him off the ball and dribbling less in Game 3, used more as a screener.

He was still rendered all but invisible — like the Nets’ chances. He shot 6-for-11 (better than the combined 13-for-41 in the first two games) and had eight rebounds and eight assists, but still turned the ball over five times.

“I think [Saturday was] not a must-win, but every game is a must-win in the playoffs so I don’t necessarily subscribe to that,” Nash said before the game. “You don’t go in and say Game 1 is not a must-win. You’re trying to win every single game. So our mentality shouldn’t change; we’re trying to win every game.”

They may only have one more, with Game 4 on Monday being an elimination game.

Jayson Tatum poured in a game-high 39 points for Boston and was serenaded by chants of “MVP! MVP!” from green-and-white clad Celtics fans in Brooklyn. Jaylen Brown added 23 points.

Nash tried a host of tweaks and adjustments, desperate to change the series momentum. Thee Nets liberally sprinkled in some zone early, and even dusted off Blake Griffin late.

Wit the Nets trailing 75-69 with 1:27 left in the third, Nash gave Griffin his first minutes since April 2. He scored eight points and gave the Nets a spark, but not nearly enough.

A star-studded crowd that included Spike Lee, Mary J. Blige, Trevor Noah, Gleyber Torres, Aaron Judge and Questlove may have watched the Nets’ last gasp.

The Nets actually led 12-5 early, thanks to seven quick points by Brown. But it didn’t last. They coughed up an 18-5 run — including seven unanswered Boston points — and couldn’t ever mount a single serious threat to even pull even after that.

After going ahead 20-19 with 4:26 left in the first on Durant’s short fadeaway, the Nets surrendered a 7-0 spurt. Boston guard Derrick White’s layup left them down by six.

The deficit reached 12 when Payton Pritchard drilled a step-back 3-pointer, making it 37-25. The clock read 9:05 left in the first half, but the contest — and perhaps the Nets’ season — was over.

The Nets did get within 51-50 on Patty Mills’ kick-out 3-pointer just before the half, but no closer.

It was 53-50 at the break after the Nets closed on an 11-5 run. But three quick turnovers in succession — two by Brown and another by Seth Curry — sparked a 7-0 run.

When Tatum found Daniel Theis for a 3-pointer, the Nets trailed 60-50 with 10:35 left in the third. They never challenged in earnest the rest of the way.