The Boston Bruins, to the strains of "God Bless America," swept the Philadelphia Flyers out of the Stanley Cup playoffs in th minimum four games with a 3-0 victory tonight.

It took maximum effort, however. The Bruins' forecheckers overwhelmed the Flyers, limiting the losers to 21 shots. Gerry Cheevers, the 36-year-old Boston goaltender, stopped all 21, recording his seventh career playoff shutout.

"He's the hottest goaltender in hockey, no question about it," said teammate Peter McNab, and nobody was ready to dispute the matter. Certainly not Philadelphia's Bobby Clarke, who three times watched in disbelief while Cheevers handled his point-blank shots.

Jean Ratelle produced the game-winning goal at 10:45 of the second period, jamming a second attempt past th Flyers' Wayne Stephenson from close range. It was merely the inevitable, as Stephenson blocked 18 first-period shots and was under heavy fire from the opening minute, when a Clarke hook started Boston off on a power play.

Don Marcotte finished the Flyers with 5:55 remaining, rebounding Bobby Schmautz' drive that had struck Stephenson's skate. Then Marcotte finshed the scoring, too, hitting an empty net with 47 seconds left in the game.

Marcotte's biggest contribution was the silencing of Flyer right wing Reggie Leach. A year ago, Leach scored eight goals despite Marcotte's constant shadowing. Leach scored none this time.

"You just hav eto skate with him," Marcotte said. "You have to keep an eye on him, but he mainly skates his wing, doesn't wander all over. The big thing we did this year was not let Clarke control the play, the way he did last year. When he got the puck, we got on him and made him pass it."

Another philadelphia noncontributor was center Rick MacLeish, a 49-goal scorer in the regualar season who also came up with a zero. MacLeish was closely watched by Gregg Sheppard and was further disoriented by injuries to his usual wingers. Gary Dorhoefer was unable to dress tonight and Bill Barber was ineffective.

This was the first time the Bruins had between the Flyers in a playoff series, having dropped a six-game final in 1974 in addition to last year's debacle. The capacity crowd of 14,597 enjoyed the turnbout, breaking into "God Bless America," the Flyers' theme of better years, with three minutes left and carrying the tune out into Causeway Street.

"Bring on the Canadiens," a sign proclaimed from the Boston Garden balcony, and it appears likely that defending champion Montreal will be the Bruins' opponent in the final. The Canadiens first must finish off the New York Islanders, whom they lead by 3-1 with the fifth game scheduled Tuesday in Montreal.

The final series cannot start before Thursday and the Bruins are willing to exchange a modicum of momentum for was some rest.

"I got my neck banged up in the first series," Ratelle said, "and we always come back every two days rest it a bit. And we've got a few other guys sore and injured. I don't mind getting a few days' rest, as long as we're in the finals."

The Flyers have a lot longer to nurse their wounds, and they aren't the least bit happy about it. Who could have dreamed that the cup euphoria of 1974 and 1975 would be followed by a four-game final wipeout in 1976 and a four-game semifinal defeat in 1977?

"I can't believe we beat them in four straight," said Boston coach Don Cherry. "I thought it would go six. Four straight - you can't do much better than that."

Indeed, the Bruins could not have played much better hockey. In the last 20 minutes, with Philadelphia struggling for survival, the Flyers were limited to four shots on goal.