The Celtics are angry. At the Knicks. At themselves. But mainly at the Knicks. While they were being blown out at home Friday, the Celtics felt the Knicks were showboating, rubbing Boston’s collective nose into the same floor they wiped with the Celtics team. “Yeah, we did. We all did. That’s what made us so mad, so angry,” guard Avery Bradley said before the Celtics’ film session yesterday. “We just got to go out there and play the right way. We’ve got to not let that happen again. After the game, that was definitely something [discussed]. Somebody said it right after we got in the locker room: ‘Oh, they’re showboating on us, they’re dunking.’ “And everybody kind of looked like, ‘Yeah you’re right,’ ” Bradley added. “But all we can do is go out there and play the right way and play hard and don’t give them a chance to showboat.” Easier said than done. Entering Game 4 at TD Garden today, the Celtics have been an offensive mess. They have averaged 75.0 points in the series, not once reaching 80. They’ve shot 39.5 percent and averaged 17.0 turnovers to negate the good of their defense. So yeah, they’ve angry. They were ticked when J.R. Smith stared at the crowd and smiled after dunking for a 21-point lead. They weren’t pleased when Carmelo Anthony laughed at Brandon Bass after the Celtics swiped at the ball. They hardly were thrilled by Smith throwing an elbow at Jason Terry that resulted in an ejection. “No comment,” Terry said about the elbow, later relenting and saying “no question” it will motivate Boston today. But Terry, who was a surprise Game 3 starter, acknowledged New York’s right to showboat. They have dismantled Boston in every area and “no doubt” acted like they have the series wrapped up. “There was some showboating going on a little bit, which I don’t care for,” Terry said. “But they have every right to do so. They’re up 3-0. There’s no pressure on them. They can come out [today], show up and they think they’ve got it won.