There have been two undeniable truths about the Miami Heat this season.

They must make 3’s to win.

They aren’t invincible with sizable leads.

The Boston Celtics have scouting and analytics teams that undoubtedly know these trends. But, really, so would anyone who simply can read a boxscore.

Take away Miami’s 3’s, and the Heat are easier to beat. The Celtics proved that again in Game 5 of the Eastern Conference finals when they extended their season with a victory — and will aim to take the same tact Sunday night when they meet the Heat again in another must-win for Boston.

“They’re going to hit some shots, they’re going to make some plays,” Celtics forward Jayson Tatum said. “They’ve got some good players. We’re just trying to make it as tough as we could.”

The Heat have played 87 games this season and shot below 20% from 3-point range in just three of them — one of them being Friday night, when the Celtics prevailed 121-108 to cut Miami’s lead in the series to 3-2.

Miami was 7 for 36 from deep, just 19%.

“Regardless of whether it’s going in or not, that can’t affect your commitment on the other side of the floor,” Heat coach Erik Spoelstra said. “And it felt like it did.”

The Celtics confined most of their Saturday plans to a film session; the Heat were doing the same along with some optional workouts. Heat center Bam Adebayo, who blamed himself for the Game 5 loss despite teammates saying otherwise, said he would spend some of Saturday on the floor looking for answers.

“This team has good resolve,” Celtics coach Brad Stevens said Saturday. “I thought we showed that last night. We’ll have to continue to show the ability to be able to handle good and bad throughout a game.”

Miami is 55-32 this season, and 18 of those losses have come in games where the Heat held a double-digit lead. Boston has beaten Miami four times this season, rallying from at least 11 points down in three of those games — including a pair of 12-point comebacks in this series.

Even though the disappointment was clear Friday night, the Heat still understand where they are: a No. 5 seed, one that didn’t even make the playoffs last season, one win from the NBA Finals. Miami needed two tries before ousting Milwaukee in the second round, saying then it learned at what level a team needs to be to win a closeout game.

The Celtics provided them another reminder of that Friday night, when they outscored Miami 70-50 after halftime and erased a 12-point second-quarter deficit.

“As you go on, the wins get harder and harder,” Heat guard Duncan Robinson said. “And doing what we want to do and advancing from this round is going to be the hardest thing we’ve done all season and our in our athletic careers for many of us. Fortunately, we have coaches and guys that have been there and know what it takes.

“But this is certainly a reminder — to think that we were just going to have a good first half and just kind of coast to a victory in this stage of the playoffs, we’re misguided for thinking that.”