Then the mistakes started.

Soon, it was the Miami Heat taking the lead in this championship series — and ensuring the Thunder would spend two days with the memories of so many things going wrong at the wrong times.

A night of missed free throws, more foul trouble for Kevin Durant, wasted opportunities in critical situations and one dreadful stretch to end the third quarter all added up to a loss for the Thunder on Sunday, falling 91-85 to the Heat in Game 3 of the finals. Miami now leads the best-of-seven championship series 2-1, with Game 4 back on its floor Tuesday night.

“A very intense playoff game,” Thunder coach Scott Brooks said. “I thought we did some good things, corrected some things going into this game, and I thought we gave ourselves a chance to win. That’s what you want to do on the road. The game was basically anybody’s ballgame that fourth quarter.”

True. Except it probably shouldn’t have been that way.

Down by eight in the early going, the Thunder clamped down defensively to change the complexion of the evening, taking away the easy paths to the basket that Miami enjoyed in the first half and forcing the Heat to shoot from outside. It simply couldn’t have worked better for the Thunder, with Miami missing 11 straight jumpers in one stretch and finishing 5 for 31 on shots taken from outside the paint for the game.

The Heat needed help to win. And the Thunder accommodated them, with mistake after mistake.

“This is not over,” said Durant, the league’s scoring champion. “It’s not over.”

He repeated the words for emphasis. But if the Thunder repeat those mistakes, then it might be over.

“Game 4 is crucial,” Thunder guard James Harden said. “It’s the series. We don’t want to be down 3-1.”

Durant finished with 25 points for the Thunder, who got 19 from Russell Westbrook and 10 points and 12 rebounds from Kendrick Perkins.

LeBron James led everyone with 29 points and 14 rebounds for Miami, which got a 25-point, seven-rebound, seven-assist effort from Dwyane Wade and 10 points and 11 rebounds from Chris Bosh.

“This team is not going away,” Heat forward Shane Battier said of the Thunder. “This is an evenly matched series. Literally every game has been decided by four or five possessions. We just want to be on the right side of those possessions when it counts.”

Like they were on Sunday night.

Derek Fisher’s four-point play with 4:33 left in the third quarter capped an 18-5 run by the Thunder, who rendered most of the 20,003 people in the building silent during that stretch. Oklahoma City led 64-54, the scoreboard showing those numbers just 68 seconds after Durant went to the bench with four fouls and a mere 28 seconds after Fisher checked in to give Westbrook a rest.

So the best two Oklahoma City players were out of the game.

If the Heat were going to turn things around, it had to be in that moment. This is where that aforementioned help enters the picture. Battier and James Jones scored six Miami points 22 seconds apart — after both got fouled trying 3-pointers, on a night where the Heat could barely make anything from outside.

James got a 3-pointer to fall late in the third, putting Miami on top 69-67. There were two lead changes in the fourth, but in the end, the Heat had enough. And afterward, Durant and Westbrook got the questions about their absences in the third quarter.

Westbrook, on why he was out: “Coach’s decision. Got to live with it.”

Durant, on why he was out: “That’s coach’s call. … Me going out of the game because of fouls is kind of tough.”

So then the coach explained his thinking.

“You’ve got to rest the guys sooner or later,” Brooks said.

There was so much for the Thunder to lament when this one was over. Durant was in foul trouble, again — a rarity for him. The Thunder were 15 for 24 from the foul line, after leading the league in that department during the regular season. Oklahoma City gave Miami eight points off turnovers in the fourth quarter — plus scored only 10 points off Miami’s nine turnovers in that period.

And it all came in a game where Miami shot 10 for 30 in the second half — though the Heat were 22 for 24 from the foul line after halftime.

“We could have been better, of course,” Durant said.

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