The New York Rangers have lost the cohesion that carried them to playoff victories over Los Angeles and St. Louis. The reason is obvious: the New York Islanders.

"They've frustrated us so much they've got us playing as individuals," Ranger captain Barry Beck said after the Islanders won Saturday night, 5-1, to gain a 3-0 lead in the best-of-seven Stanley Cup semifinal. "We're not playing as a team and we're making it tough on ourselves. We're trying but we're not accomplishing much.

"They're making us play that way. They're playing damn good hockey. We have to come back Tuesday and control the puck, get the feel of playing as a team. But they don't give you much. They're finishing their checks and they aren't making many mistakes."

If Beck seemed less than optimistic, it was with good reason. He is the heart of the Ranger team and he is hurting. In the third period Saturday, Beck tried to check Islander Wayne Merrick and lost his balance. He spun into the boards, lay on the ice for a few seconds and rose slowly.

Although he did not miss a shift, Beck had an Ace bandage wrapped around his midsection afterward and was in some pain.

"I just snapped my neck," Beck said. "Then when I got up my neck was all right, but I pulled something in my back. I'll be all right Tuesday."

Ranger Coach Craig Patrick refused to concede defeat, saying, "They've won three in a row and there's no reason why we can't win four in a row."

A Ranger fan sitting behind the press box, who had joined the Madison Square Garden majority in screaming obscenities at Islander captain Denis Potvin during the first two periods, probably had things in better perspective. "Well, four more periods and it's over," he said between periods with the Islanders leading, 4-0.

The Islanders declined to accept victory early and Potvin said, "the series isn't over. We should know that better than anyone. We're the team that came back from 3-0 (in games) twice. But if we get a performance Tuesday like we got tonight from 20 guys we'll win."

In 1975, the Islanders trailed Pittsburgh, 3-0, in the quarterfinals and won four straight. Then, in the semifinals, they were down, 3-0, to Philadelphia and won three in a row before falling in Game 7.

Ranger fans knew their heroes would need a lot of luck to topple the powerful Islanders, but they hoped at least to win a game or two in the Garden, where they had beaten the Islanders five straight times and had lost only once in their last 17 games.

Saturday the Rangers started out fast, outshooting the Islanders by 6-1 in the first seven minutes and killing off a penalty in the process. But, as so often has happened to the Washington Capitals, a Ranger power play took away the momentum.

With Clark Gillies off the holding, the Rangers' extra-man maneuvers were so inept and disorganized that they obviously deflated both Rangers and fans. Then Islander Bryan Trottier flattened Beck with a stunning check and the Islanders were in control.

Power-play goals by Mike Bossy and Bob Bourne gave the Islanders a 2-0 lead and the circumstances of Bossy's score were enough to unsettle Ranger optimists.

Bossy, cutting across the slot from the rightwing corner, tried to pass to Trottier near the left post. Ranger defenseman Carol Vadnais blocked the pass, but the puck caromed off his skate and slid inside the right post as goalie Steve Baker, moving to counter Trottier, could not get back.

Goals by Ken Morrow and Bob Nystrom made it 4-0, Nystrom hitting five seconds before the second period ended as Ranger Chris Kotsopoulos tried to pass around behind his net and the puck hit a protruding board, caroming out to Wayne Merrick, who fed Nystrom.

The Islanders' power play, No. 1 in the NHL during the regular season, is 24 for 61 in the playoffs, five for 17 in this series. The Rangers, 20th in that department during the year, are nine for 54. In this series, the Rangers are two for 15 and have yielded two short-handed goals.

The Rangers are at their best when they are hitting and some of their hitters are awkward, taking frequent penalties that have reached a club-record 392 minutes during these playoffs. With the Islanders' power play so potent, the Rangers have been forced to be more cautious and the results, so far at least, have been disastrous.