Three U.S. physicians, alarmed over talk of nuclear war and believing that neither Americans nor Russians fully comprehend the medical consequences of such a war, held a 90-minute private meeting here yesterday with Soviet Ambassador Anatoliy Dobrynin.

Dobrynin gave the doctors, representatives of a much larger group known as "Physicians for Social Responsibility," a personal letter from Soviet President Leonid Brezhnev endorsing their efforts to alert people to the nuclear dangers.

"Such explanations," Brezhnev wrote, "will further strengthen the will and activity of those who come out for stopping the arms race" and, he added, "for maintaining normal relations between all the countries including, of course, the U.S.A. and U.S.S.R."

The doctors said they were aware that the Brezhnev letter might be viewed as Soviet propaganda, or part of a Soviet "peace offensive" after the invastion of Afghanistan. They also said Dobrynin was aware that it might be construed as such here.

Nevertheless, Dobrynin said the Soviets "attach the utmost importance to this movement" by the U.S. physicians and that Moscow "would be very responsive" to various proposals made by the group, according to Dr. James Muller, a cardiologist at the Harvard Medical School.

Muller, aware of the problem of being caught in a potential propaganda situation, said in an interview that "we don't want to get into the politicar arena. Our strength isn't there. We are doctors who know what a burn patient looks like and we want to tell the people of the United States and Soviet Union what 100,000 burn patients look like."

A Carter administration spokesman, asked for comment on Brezhnev's remarks, said "in the administration's view, conduct, not words, weighs heaviest and it is mutual restraint in international conduct that is the key ingredient to the maintenance of normal relations . . . and to the preservation of peace."

In February, about 650 doctors, medical personnel and private citizens attended a two-day conference in Cambridge, Mass., devoted to the idea that nuclear arms had become even more dangerous because people don't think enough about the real consequences of their use.

Early this month, several hundred medical specialists and private citizens also signed and published an open letter to President Carter and Brezhnev. It was the response to this by Brezhnev that the group received today.

Along with the letter, Dobrynin told the group in a separate statement that both Brezhnev's reply and the group's open letter would be published today by the Soviet news agency Tass. A reply by Soviet medical experts will also be published soon in the government newspaper Izvestia and the Soviet Medical Gazette.

A proposal by the U.S. group to hold a joint conference with Soviet scientists on the possible medical consequences of thermonuclear war also "has received a positive response in the U.S.S.R.," the statement said.

Muller says the idea is to include doctors from Japan, the only country to experience a nuclear attack, in an annual conference. "We feel the arms race is probably going to go on indefinitely," Muller said. "So we are trying to inject a new element into arms control and that is to increase public awareness of the medical consequences of that arms race and nuclear war."

The meeting here today, and the prospect of a joint conference, was one of the rare exchanges of a scientific nature between Americans and Soviet officials since the invasion. Since then, many U.S.-Soviet scientific meetings have been canceled.

The statement provided by Dobrynin yesterday, however, also said that if sponsors of the U.S. open letter "happen to be in Moscow" Brezhnev "would have them received at the highest level." Dr. Bernard Lown, a professor at the Harvard School of Public Health and a major figure in the organization, is in Europe now and Dobrynin reportedly said a visa would be arranged for him to go to Moscow.

The organizers had met earlier this month with White House counsel Lloyd Cutler, who expressed Carter's support of their aims but who also reportedly asked for the group's support for the new U.S.-Soviet strategic arms limitation treaty that has not been ratified by the Senate.

Yesterday, the doctors said Dobrynin also called for SALT to be ratified so that discussions could proceed with a third round of arms agreements.

In his letter, Brezhnev said he shared the group's concern "for the fate of mankind in connection with the danger of nuclear war. Since the time when atomic energy was first used for military purposes the Soviet Union consistently stands for banning these and all other types of weapons of mass destruction and annihilation."

The group also discussed the recent suggestions by the State Department that a Soviet accident with outlawed germ warfare weapons may have killed hundreds of people near Sverdlovsk.

Dobrynin reportedly claimed the incident was caused by infected meat from a private, rather than state, farm, that it was publicized in the Soviet Union a year ago and wondered why the United States didn't say anything about it sooner.