By Lewis Gould

Memo to the Sixers:

This what the brink looks like.

Al Horford hit the game-winning layup with 5.5 seconds left in overtime as the Celtics beat the Sixers 101-98 to take a 3-0 lead in an Eastern Conference semifinal series.

The 76ers called timeout, after Horford’s basket.

On the ensuing inbounds play, Horford stole the pass by Ben Simmons and made two free throws. Marco Belinelli missed a desperation 3-pointer as time expired to seal the result.

“We’re trying to find a way, and this group is playing hard every night,” Horford told ESPN after the game, referring to the Celtics’ deep playoff push without injured stars Kyrie Irving and Gordon Hayward.

Boston won the first two games of the series at home, beating the Sixers 117-101 in Game 1 and 108-103 in Game 2 on Thursday. The Celtics are 36-0 in franchise history when leading a playoff series 2-0.

No team has ever won a best-of-seven series after trailing 3-0.

Game 4 is tomorrow night at the Wells.

Jayson Tatum led the Celtics with 24 points, Terry Rozier had 18 points, Jaylen Brown scored 16 and Horford finished with 13. Tatum became the first Celtics rookie ever to score 20 or more points five times in the postseason.

Joel Embiid had 22 points and 19 rebounds to pace the 76ers. JJ Redick scored 18, and Simmons had 16 with eight rebounds and eight assists after scoring only one point in the Game 2 defeat.

The Celtics snapped a six-game road losing streak dating back to March 28 against the Utah Jazz in the regular season, including all three games in their seven-game, first-round series against the Milwaukee Bucks.

Simmons’ driving layup with 1:05 remaining in overtime put Philly up 98-94. Tatum brought the Celtics within two on his layup with 55.4 seconds left.

Horford made one of two free throws with 42.5 seconds left to bring Boston within a point. Simmons missed a layup on the other end with 17.2 seconds remaining, and Boston’s Marcus Morris came down with the rebound.

With the game tied at 85 entering the last minute of regulation, Belinelli was fouled and made two free throws to put the 76ers ahead by two with 41.2 ticks to play.

Brown tied it on a layup with 24 seconds left. He gave the Celtics an 89-87 lead after Rozier came up with a Redick turnover and fed Brown on a two-on-one break with 1.7 seconds to play.

Belinelli’s turnaround corner jumper as time expired sent the game to OT. His foot was on the arc, so the shot merely tied the game instead of winning it. The confetti drop was accidentally triggered after Belinelli’s jumper, leading to a lengthy delay to clear off the court before the start of overtime.

Philly led 51-48 at halftime.