ATLANTA — The Knicks had finally found a solution to slowing down CJ McCollum.

But for the most important defensive play of the game, they went away from it.

Jalen Brunson started out guarding McCollum during the Knicks’ 109-108 Game 3 loss to the Hawks on Thursday night at State Farm Arena.

But, just like the first two games of the series, McCollum toyed with him.

Miles McBride and Jose Alvarado each got stints trying to slow McCollum down in the second quarter, but the results were the same.

It wasn’t until the Knicks had Josh Hart guard McCollum in the second half that anything changed. McCollum was just 2-for-5 when guarded by Hart, per the league’s official tracking stats.

Needing just one stop to all but secure their comeback, though, the Knicks went back to McBride. And it backfired.

With 16.4 seconds left, the Knicks led by one and the Hawks had a side out-of-bounds in the frontcourt. Coach Mike Brown went with a small lineup, without Karl-Anthony Towns or Mitchell Robinson on the floor.

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That meant OG Anunoby guarded Onyeka Okongwu, Mikal Bridges guarded Nickeil-Alexander-Walker and Landry Shamet guarded Jonathan Kuminga. As a result, Hart was on the inbounds passer — Jalen Johnson — and McBride was on McCollum.

McBride quickly fell a step behind McCollum, who curled as he caught the inbounds pass.

McBride eventually caught up, but it was too late — McCollum comfortably pulled up for a midrange jumper. McBride got a hand up, but his lack of size compared to Hart meant it didn’t affect McCollum, who easily buried the shot that ended up being the game-winner.

“I knew they had to go quick,” McBride said Friday. “Main thing is, he’s a really good player. I’m not happy with my contest. He made the shot. I feel like if I maybe had bumped him earlier, but trying to get a feel for how the game was being called. I don’t want to put him at the free throw line in a sense like that.

“Maybe just be more aggressive and make it tougher.”

The Knicks have made McCollum — who has never been an All-Star — look like a superstar this series. Their inability to contain him has been among the biggest problems this series.

Hart stopped the bleeding for a period of time. “Just tried to make it tough for him,” is how he described it.

But he didn’t get a chance to finish the job.

When it mattered most, McCollum again had the last laugh. And, as McBride said on Friday, the Knicks are now “playing for our lives.”

With their lives on the line, who Brown has guarding McCollum is at the top of his most critical decisions.