Three American physicians took part in an hour-long discussion on the medical effects of nuclear war broadcast in prime time on Soviet television to about 150 million viewers last night.

The extraordinary, unedited debate, which also involved three prominent Soviet physicians, focused on immediate and long-term effects of nuclear explosions, the economic and psychological costs of the arms race and the growing danger of accidental war, Washington Post correspondent Dusko Doder reported.

The consensus reached by the international panel was that "there can be no winner in a nuclear war," because it would destroy not only hundreds of millions of people but also human civilization itself.

The Soviet audience for the first time heard fresh views on the touchy issue of civil defense when Dr. Bernard Lown of Harvard's School of Public Health asserted that civil defense efforts against nuclear weapons were not only futile but also constitute a "negative factor that can contribute to a nuclear war."

Nobody refuted Lown's views, despite the fact that Soviet military authorities advocate civil defense programs.

Along with Lown, who is leader of Physicians for the Prevention of Nuclear War, the other American participants were Dr. James Muller of Harvard University Medical School and Dr. John Pastore of Tufts University School of Medicine.