It took Mike Bossy's third goal of the game, with only 2:46 remaining, to write the epitaph to the Washington Capitals' first challenge for the Stanley Cup.

The Capitals, seemingly headed for a humiliating exit when they trailed the New York Islanders by three goals in the second period last night, battled back within one on scores by Bobby Gould and Ken Houston. Then Bossy, limited in the first three games of the series, converted Anders Kallur's pass and John Tonelli followed with an empty-netter as the Islanders won, 6-3.

The Islanders, bidding for a fourth straight Cup, will go on to play the New York Rangers for the Patrick Division championship after ousting the Capitals, three games to one, in their best-of-five series.

Only one post-1967 expansion team, the Islanders against the New York Rangers in 1975, has been successful in its first playoff competition. Washington joined Buffalo, Vancouver, Atlanta, Colorado, Winnipeg, Hartford, Edmonton and Quebec as a first-time loser.

"We had a tough road starting against the three-time Stanley Cup champions, but we never quit," Houston said. "This team never has quit. Objectively, it was a learning experience. We have the nucleus to be a hell of a hockey team in the next few years."

Gaetan Duchesne, whose first Stanley Cup goal created a 1-1 tie early in the second period, said, "We can keep our heads up. It was a good season, just two points behind them. It's very tough, because they're the champions, and even though we make few mistakes, when we make them, they take advantage. They have the experience. Now we have some and I know I learned a lot this series."

There were two occasions when the Capitals could have packed it in last night. The first came after three goals in less than four minutes by Bob Bourne, Bossy and Mats Hallin lifted New York into a 4-1 lead. The second followed a two-minute, two-man Washington advantage late in the second period when New York goalie Roland Melanson turned aside eight shots.

Instead of bowing to the inevitable, the Capitals struck back. Just 27 seconds after the Islanders' two penalties expired, with the first boos emerging from the sellout crowd of 18,130, Gould deflected Brian Engblom's shot from the right point for his fifth goal of the series.

Less than a minute later, as Duchesne tried to split the New York defense, he was hooked down by Denis Potvin. That set up Washington's fourth and final futile power play of the game and it took a fine stop by Al Jensen to avoid a shorthanded goal by New York's Billy Carroll.

Pat Riggin replaced the weary Jensen at the start of the third period and made a sensational stop on Bossy, diving out to knock the puck off his stick.

So there was still only a two-goal difference when Houston blocked a shot by Potvin, scooped up the puck and skated in on a breakaway, shooting it between Melanson's legs. With 8:26 remaining, it was 4-3 and the decibel level was raising the roof.

A brief silence, no doubt inspired by disbelief and disappointment, greeted Bossy's score from the slot that assured Washington's elimination.

Then there was a surge of applause, hardly interrupted by Tonelli's empty-net score with 1:11 left, and it followed the Capitals to their dressing room.

"They scored the timely goals and put it away when they got a chance," Engblom said. "They stopped us when we had some outstanding chances. But just the distance between the two penalty box doors was the difference in their tie-breaking goal."

Bourne put New York ahead to stay at 3:13 of the second period, snapping a 1-1 tie on a breakaway out of the penalty box, where he and Engblom had gone for two minutes after a brief scuffle. At the time, it appeared that referee Andy Van Hellemond had given Washington a break by limiting the sentences to minors, when Engblom might have been lost for five minutes. It did not turn out that way.

The visitors' penalty box is a few feet closer to the Washington net than the home box and Bourne used his head start to grab Duane Sutter's pass and outdistance Engblom before putting the puck behind Jensen.

Bossy made it 3-1 on another breakaway, after Bryan Trottier had knocked the puck off Rod Langway's stick. Then Hallin deflected Mike McEwen's shot and things looked very dark for the Capitals.

"Again we had a lapse in the second period and it put us in a position where we had to struggle and fight and try to dig ourselves out of a hole," Coach Bryan Murray said. "It's tough to do against this team. We silenced Bossy for three games and then he killed us. The other night it was Trottier. There's no question the Islanders are the quality team in our division."

New York had an opportunity to blow the game open in the first period, but the Capitals' penalty-killing unit hung tough during 5 minutes 48 seconds of disadvantage.

First, Randy Holt was chased for holding at 13:14. Then, with 12 seconds left on Holt's penalty, Scott Stevens began punching Kallur, who turned away and went unpenalized, while Stevens was charged with a double minor.

During the brief two-man shortage, Bossy hit a post. As Holt stepped on the ice, Jensen blocked Denis Potvin's shot and smothered Trottier's rebound.

Jensen made three more saves before the expiration of Stevens' first penalty and, since Langway and Engblom had stayed on the ice throughout the two minutes, Murray summoned Riggin to relieve Jensen. Riggin was permitted a warmup, giving the defensemen a chance to rest.

After Riggin gloved a drive by Trottier 18 seconds later, Jensen returned and made a couple of fine saves before Stevens returned to the ice.

The Capitals' defensive problems were compounded by the absence of Timo Blomqvist, unable to play because of a rib injury, and a bruised knee suffered by Langway when he was flattened in the second period.

"I got hit in the top of the knee and it swelled up and got stiff," Langway said. "I wasn't 100 percent in the third period."

Langway, like his teammates, gave 100 percent anyway.