Kevin Garnett, arguing a second-half call with a referee in Game Three against the Hawks in Atlanta on Saturday night, and the Celts must regroup.

The Providence Journal / Glenn Osmundson

ATLANTA — A few hours before Game Three of his team’s playoff series against the Atlanta Hawks, Kevin Garnett labeled the game the biggest of the season for the Boston Celtics.

That may have been the case Saturday, but the tag is even more appropriate heading into tonight’s Game Four. If Atlanta shocked and surprised anyone with its 102-93 win over the Celts, the reaction will be much different if the Hawks can tie the series at two games apiece by winning again tonight at Philips Arena.

This is truly a swing game. Lose, and all bets are off for the Celtics. Maybe they are not a dominant team, one that some observers thought would lower the boom on defense and ride the scoring skills of the New Big Three through the Eastern Conference with relative ease.

A Celtics win would make Game Three seem like a blip on the radar. It would also give the Celts a 3-1 series lead heading back to the TD Banknorth Garden for Game Five on Wednesday.

Despite two dominating wins in Boston, right now the Celtics haven’t done a whole lot in the series, according to coach Doc Rivers.

“I didn’t think they were going to go away. They shouldn’t go away,” he said. “You’re in the playoffs and at home. No one has done anything yet in this series. There’s been three games played and the home team has won (all of them).”

Rivers and the rest of the coaching staff chose to rest the players’ legs yesterday, but put their eyes through a workout during one of the longest film sessions of the season.

“Obviously, it was a long one,” said Rivers. “We just felt we couldn’t get anything by going out on the floor. We needed to watch film and just look.”

Rivers said he stayed up until nearly six o’clock yesterday morning going through the game film. He saved many of the lowlights for his players.

“Offensively, we were walking to our spots. No sense of urgency. No picks. Cuts were soft,” he said. “Defensively, very much the same.”

How did the players handle a film session that frequently resembled a horror flick?

“It was terrible. They say film doesn’t lie,” said Garnett. “It was productive and it was good. It put everything into perspective.”

While the Celtics said they weren’t surprised that the Hawks played so well, they were clearly shaken by the way they went about their own business. Most of the things that went so smoothly in the first two games turned dramatically in Game Three. The Celtics outrebounded Atlanta by nine over the first two games but were blitzed (43-35) on the glass in the third game. The Celtics’ defense limited Atlanta to 38.2 percent shooting and an average of 79 points in Boston, but the Hawks shot 47.4 percent and had 79 points with three minutes to play in the third quarter Saturday night.

Perhaps more depressing was the way the Hawks put it on the Celtics. High-flying forward Josh Smith (27 points, 9 rebounds) slammed home dunks not seen in these parts since Dominique Wilkins was creating highlights, and rookie Al Horford (17 points, 14 rebounds) schooled Boston’s Kendrick Perkins and woofed at Paul Pierce in the closing minutes.

“No. 1, they aren’t going to go away. We have to play,” Rivers said. “Number two, we just didn’t do things in our team character.

“They played aggressive, they played hard,” Rivers added. “We talk about the 50-50 thing where every time there’s a loose ball there’s a 50-50 chance that either team can get the ball. I don’t think we got one. It might’ve been a 90-10 thing last night. On the road you have to win that war and they won it. They won it with ease.”

As the players exited a conference room at their hotel, they walked past a small group of press members. No one appeared very happy. Others were downright curt. But with such a veteran team, no one appeared shaken by the first playoff loss for a franchise that hasn’t won a playoff series since 2003.

“I feel very confident about what we need to do to win the ball game,” said Pierce. “I know we can’t play as bad as that two games in a row.”

kmcnamar@projo.com