After dissipating three leads of three goals or more against the Philadelphia Flyers during the regular season, the New York Rangers have found the formula for flagellating the Flyers while comfortably ahead.

The Rangers' problem now is to insure that dreams of Islanders do not distract them from the job at hand tonight in Philadelphia, stretching their 3-1 margin in games to the four victories necessary for advancement to a semifinal confrontation with the Long Island upstarts.

The cretins who inhabit the Upper reaches of Madison Square Garden altered the targets of their usual obscenities from Flyers to Islanders during the final period of the Rangers' 6-0 victory Sunday night.

Although the 18-2 margin over the last three games would indicate the Flyers are ready for the coup de grace, Ranger Coach Fred Shero is aware that the remnants of the team he coached to two Stanley Cups will not surrender without a fight.

"In playoff hockey, nothing surprises me after 21 years," Shero said. "One night when I was with Omaha, we lost 11-0, because the puck was going in for the other team. The next night we won, 1-0, in the same building.

"We have to keep on checking the Flyers and not let them up. You just don't skate in the big leagues and win. If you just skate with the Flyers, they'll kill you."

The pucks have certainly been going in for the Rangers and goalies Robbie Moore and Wayne Stephenson, furnished little help by inept defenders, have had such tough times that Coach Pat Quinn is considering a shift to Rich St. Croix tonight.

New York will continue with John Davidson, who had been a magician in compiling a 1.14 goals-against average over six playoff games. Davidson has been helped by some crazy bounces off crossbars and posts, which just add to the team's overall confidence.

"This is the first time in two years we've had breaks like this," Davidson said. "Maybe we're working hard enough that we deserve the breaks."

"I was with the Flyers for a couple of Stanley Cups and we had a couple of good bounces," Shero said. "You have to be careful. The puck can start bouncing the other way."

Shero's departure from Philadelphia last spring, to become the Ranger's general manager and coach, has injected considerable emotion into this rivalry. Shero has been careful not to pour on additional lighter fluid.

The Flyers' frustration prompted them to resort to questionable tactics in the third period Sunday and one particularly foul play, by Paul Holmgren, sent Ranger captain Dave Maloney to the hospital with a severe charley horse. Yet when it was suggested that Philadelphia was "chippy," Shero cast no stones in the vicinity of his old glass house.

"I think they were aggressive, but I wouldn't call it chippy," Shero said. "They have to be aggressive to win. They were using the body but I didn't see them using the stick tonight. Maybe I missed something."

He didn't as the sudden appearances of Ranger policeman Nick Fotiu following especially rough movements have proven during the series. But there will be no "Shero Says" quotes circled in red in the Flyer dressing room to help the Philadelphians off the floor. They must practice self-motivation.

Montreal, Boston and the Islanders are soothing their bruises following quarterfinal sweeps while awaiting the conclusion of this series. Boston and Montreal, Stanley Cup finalists the last two years, will meet in the semifinals this time.

The Canadiens finished off Toronto Sunday on a sudden-death goal by Larry Robinson. Approriately, NHL penalty king Tiger Williams was serving a rare overtime violation when Robinson scored.

The Islanders extended Chicago's record playoff losing streak to 16 games with a 3-1 decision and Boston polished off Pittsburgh, 4-1. CAPTION: Picture 1, Coach Fred Shero (above) leads Rangers into his former bastion in Philadelphia, seeking kill in Stanley Cup quarter final. Picture 2, Philadelphia Flyers and New York Rangers mass in front of goal during 6-0 Ranger victory in playoff action late in game. AP