PHILADELPHIA — Late in the third quarter of Game 3, all of James Harden’s struggles came into focus.

As the shot clock was winding down, the 76ers guard tried to find space to take a shot, but Jaylen Brown closed down all of his potential avenues. Harden ultimately took a tough fadeaway jumper that Brown contested well. When it missed, Brown leaked out for an outlet pass and took it all the way to the basket and drew a foul on Harden as he completed a three-point play.

Harden crashed to the floor on his back as the Celtics took an eight-point lead. The sequence exemplified how everything has gone wrong for Harden and the Sixers. After he scored 45 points to will Philadelphia in its Game 1 victory, Harden has become a complete non-factor over the last two games with Joel Embiid back, and the biggest reason why the SIxers have faltered.

Harden scored 12 points on 2-for-14 shooting – including 0-for-6 from deep – in Game 2 and followed that up with a 3-for-14 effort with five turnovers in Game 3. Though he finished with 11 assists, he seemed hesitant as he passed up looks he would typically take. His struggles were so noticeable that it drew the ire and boos of 76ers fans at Wells Fargo Center.

“I got to watch the game, but I’m pretty good on basketball instincts, I know when to score and I know when to pass,” Harden said during a short media session after Game 3. “So I’m pretty sure a lot of them was the right play.”

Harden had a one-word answer when asked if the Celtics did anything to impact him.

“Nope,” he said.

But Boston has certainly made a more concerted effort to lock down Harden since his Game 1 performance. And it’s been Brown who has led the charge. In Games 2 and 3, the 76ers won both opening tips. In both instances, Brown has picked up Harden full-court right away. It’s illustrated how much focus the Celtics have put on Harden. They let him get too comfortable in Game 1, and they haven’t since.

“I don’t know, just having pressure, intensity,” said Brown, who wanted and asked for the Harden assignment. “Just coming out and playing. He’s a great player and just trying to make it tough on him. That’s it. …

“Just doing my job, playing defense, having fun and we just got to keep doing it and get ready for the next game.”

If the Celtics can frustrate and shut Harden down again in Sunday’s Game 4, they should be heading home with a 3-1 series lead. They know the recipe that works.

“We’re just trying to make him work,” Celtics coach Joe Mazzulla said. “He’s obviously a great player and can score at all three levels. And so we’re just trying to make him work as much as we can, trying to keep him off the free throw line. So guys are just working to take away those details.”

For the last three years, Grant Williams’ parents have been complaining to him about not wearing a mouthpiece. He wore one in college and during his rookie season with the Celtics but then ditched it.

“Let’s just say last night didn’t help,” Williams said on Saturday.

There’s a good – or painful – reason why, after the 280-pound Embiid inadvertently stomped on the back of Williams’ head during a scary collision late in Game 3 that could have been worse. Williams face-planted on the hardwood, leaving him bloodied with a swollen nose. He got three stitches on his mouth after the game with the help of the 76ers’ team dentist, who drove him 20 minutes to his office where he got the stitches and got fitted for a mouthpiece.

“I was thankful for him offering and he did a phenomenal job,” Williams said.

Williams said he didn’t need to take a concussion test from the stomp, and all things considered, he said he was happy that he was relatively unscathed from the incident.

Williams said he had several people reach out to him, including his family, his friend and actor Mark Wahlberg, and his former Tennessee teammates who made fun of him.

“It was a cool little bring everybody together,” Williams said. “I was fine, just move on and get ready to play the next game.”

Robert Williams is good to go for Sunday’s Game 4 after a brief scare in the fourth quarter on Friday, when he suffered a right forearm contusion after diving out of bounds for a loose ball. He received treatment on the bench before heading to the locker room and ultimately didn’t return, but wasn’t listed on the injury report. He was wearing a sleeve on his right arm during Saturday’s walkthrough.

“Feeling good,” he said. “Little sore but I’m feeling good.” …

Marcus Smart, who’s suffered his share of injuries already this postseason, is questionable for Game 4 due to a left ankle sprain. Blake Griffin, who was out for Game 3, is also questionable because of low back pain. …

After Friday’s win, Mazzulla said he expects a war in Game 4. He knows the Celtics will likely get an even better shot from the 76ers as they fight to avoid a 3-1 series hole.

“I expect them to play better,” Mazzulla said. “I expect them to play hard, I expect them to punch, I expect them to come out aggressive. So we have to dictate that, we have to manage that, and we just have to execute.”