Twenty-one days, three matchups, three nailbiters.

After trading one-point victories on each other’s home court, the Celtics and 76ers squared off in their third thriller of the young season Tuesday night in Philadelphia.

Kelly Oubre Jr. beat Jaylen Brown for a go-ahead putback with 8.7 seconds remaining, and Neemias Queta couldn’t slam home a last-second heave by Derrick White, securing a 102-100 win for the Sixers.

Oubre’s game-winner came after rising Celtics reserve Jordan Walsh blanketed All-Star Sixers point guard Tyrese Maxey on three consecutive late-game possessions. Maxey fed a pass to Justin Edwards, who missed a 3-pointer, and Oubre caught Brown ball-watching to grab the offensive rebound.

Moments later, White threw up a 38-footer while being blanketed by Andre Drummond. Queta, left alone in the paint, grabbed the ball above the rim with one second remaining but failed to convert the would-be game-winner.

Walsh and the Celtics were able to largely contain Maxey (21 points on 5-for-17 shooting, nine assists) and rookie sensation VJ Edgecombe (six points, 2-of-11), and Philly was playing without its other headliners, the injured Joel Embiid and Paul George. But big nights from backups Drummond (14 points, 13 rebounds) and Edwards (22 points on 8-of-9, including back-to-back-to-back 3-pointers midway through the fourth quarter), coupled with woeful Celtics shooting early in the game, helped the Sixers prevail in the Atlantic Division matchup.

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Brown finished with 24 points, six rebounds and five assists, White had 18-5-7, and Anfernee Simons had 17-4-5 off the bench for Boston. Payton Pritchard managed a season-low five points on 2-of-9 shooting and was lifted in crunch time in favor of Simons, who scored 10 of his points in the fourth quarter.

The loss dropped the Celtics to 5-7. They’ll look to bounce back Wednesday night when they host the scuffling Memphis Grizzlies at TD Garden on the second night of a back-to-back.

Here were four additional takeaways from Tuesday’s defeat:

1. First-half shooting woes

The Celtics shot just 28.0% from the field and 19.0% from 3-point range during a scattershot first half. And their top scorers were the main culprits.

Brown went 1-for-7. White was 1-for-8. Pritchard attempted five shots and missed all of them. Simons: 2-for-8. That quartet also went a collective 1-for-14 from deep before halftime, with Simons providing the lone make.

Team-wide, the 76ers outshot the Celtics by nearly 20 percentage points on total first-half field goals (47.5% to 28.0%) and on first-half threes (38.9% to 19.0%). That Boston only trailed by 10 at the break, 51-41, was a testament to its effort in other areas.

The Celtics attempted 11 first-half free throws, owned an 8-2 edge in offensive rebounds, committed just three turnovers and got more positive contributions from its bench, which outscored Boston’s starters 25-16 over the first two quarters.

2. Walsh impresses again

The most impressive member of that second unit: Walsh, who again played with the type of relentless energy that’s helped him barge his way into Joe Mazzulla’s rotation after a string of early-season DNPs.

The third-year pro tallied a steal and a block in his first three minutes of floor time, and he closed out the first half with a driving dunk. He also committed an ill-advised foul on Drummond 85 feet from the basket — but his aggression in doing so prompted the veteran center to toss the ball at him in retaliation, resulting in a technical foul and a free throw for Boston.

Mazzulla chose to roll with Walsh over starter Josh Minott to begin the second half, and Walsh rewarded that trust by grabbing a hustle-play offensive rebound on the opening possession of the third quarter. Later in the same shift, he blocked Oubre in the lane, scored on a transition dunk and hit a three. (Though defense is his calling card, Walsh also is 6-for-13 from beyond the arc this season, including the late triple that sealed Monday night’s win at Orlando.)

Walsh has played real, extended, non-garbage time minutes in four straight games for the Celtics, and the team has outscored opponents by 51 points with him on the court during that span.

His late-game defense against the ever-dangerous Maxey — who dropped 40 on the Celtics on opening night — put Boston in a position to win in the final minute. Walsh’s final line: 29 minutes, eight points, seven rebounds, one assist, two steals, two blocks.

Minott did not see the floor in the second half. He was a minus-10 in his eight minutes.

3. Stars come alive in third

The Celtics’ two most established players finally shook their shooting slumps during a dominant third quarter for Boston,

Brown, held to seven points in the first half, exploded for 14 third-quarter points on 6-of-8 shooting. White, whose shooting has been uncharacteristically erratic since the season began, attempted six field goals in the frame and made five of them, including three of his four 3-pointers.

Fueled by that duo’s late-arriving offense, the Celtics outscored the Sixers 36-20 in the third, shooting 65.2% from the field and 5-for-9 from deep. They made more field goals (15) than they did in the entire first half, overrunning a Philadelphia team that ranks last in the NBA in third-quarter point differential this season.

Boston took a 77-71 lead into the fourth after Quentin Grimes beat the buzzer with a 37-foot heave.

Simons, who poured in 25 first-half points Monday against the Magic, got hot early in the fourth. But answers by Grimes and Edwards kept Philly in the game until Edwards’ solo 9-0 run put the home team back ahead with 4:42 remaining.

There were five lead changes and two ties in the final five minutes.

4. Gonzalez injured

Celtics rookie Hugo Gonzalez exited toward the locker room midway through the third quarter after spraining his ankle. The team initially listed him as questionable to return, then announced he was available, but he did not re-enter the game.

The 19-year-old wing tallied five points, two rebounds and one steal in six minutes, his shortest outing since Oct. 29.