The doubles team of John McEnroe and Peter Fleming defeated Yannick Noah and Henri Leconte, 6-3, 6-4, 9-7, today to give the Unites States an insurmountable 3-0 lead over France and assure the U.S. team its 28th Davis Cup final victory.

McEnroe defeated Noah and Gene Mayer downed Leconte Friday to give the United States a 2-0 advantage. Two additional singles matches will end the competition Sunday, though the U.S. team, captained by Arthur Ashe, has been assured of victory.

Noah and Leconte, seemingly dispirited by their singles losses, played well in only the third set when Fleming was in obvious discomfort from a chronic thigh injury.

However, the French could not capitalize on their chances, even when leading, 3-0, after breaking Fleming's service in the second game. Leconte was broken back in the fifth, and a set point was squandered in the ninth game when the young left-hander netted a backhand.

Noah dropped his service to give the Americans an 8-7 advantage, and McEnroe, who conceded only 11 points on his service in the entire match, served out the next game to end the match.

While Noah and Leconte struggled by making wild, long or low returns and other unforced errors, McEnroe and Fleming simply bided their time for most of the match and waited for an opening.

The first breakthrough came in the fourth game of the first set, when Leconte's service went to six deuces. The inexperienced Frenchman hit two successive returns beyond the sideline to drop his service and the American duo went on to wrap up the set, 6-3.

In the second set, Noah's service in the ninth game was broken to love, and McEnroe followed with a love service game for 6-4. McEnroe anchored his side consistently well and more than made up for Fleming's injury-caused inconsistencies.

Helped by the the 2-0 cushion from the opening singles, the United States secured its fourth Davis Cup victory in five years and its 28th since American Dwight Davis created the idea for the competition between nations in 1900.

Asked which of the four finals in which he has played was the most memorable, McEnroe said, "Each one is different, but this was one of the best if not the best. I played on my worst surface and won -- that was nice."

Of his partner, McEnroe said, "I guess Peter played fair to awful."

But the irascible McEnroe went on to praise Fleming. "His serve is hard enough to keep opponents guessing, and that enables me to jump about at the net -- Peter's the perfect partner for me."

Fleming himself admitted that he had been suffering from cramps during the match, and did stretching exercises at the changeovers to try to keep loose.

"Naturally, we're disappointed," Noah admitted. "But we're up against two of the world's top players. They both serve and volley so well that it's difficult to get into a match against them."

The French, who have not won the trophy since 1932, needed a small miracle to keep the final alive, and the crowd of 15,000 at Grenoble's Sports Palace rooted wildly for Noah and Leconte.

"On the whole they have been very fair," McEnroe said of the fans. "Of course, it's better for us to play in the United States. But that's what makes the Davis Cup so great, the way the crowds root for their country."

Jean Paul Loth, France's nonplaying captain, said the French would be back in future Davis Cup competition. "I was very encouraged the way my young team played," Loth said. " . . . their performances this year have given a great boost to tennis in France."

In Sunday's singles Noah is scheduled to play Mayer and Leconte will face McEnroe.

"It's a pity that it's all over before the final, but . . . we're still going to go out trying to win tomorrow," Noah said. "(A) 3-2 (score) is better than 5-0."