When one leaves, it's nothing unusual. When two leave, it's little out of the ordinary. But when three tenured professors announce in a period of two weeks that they will say farewell to Harvard, it looks like a mass exodus.

Michael L. Walzer, professor of Government, and Glen W. Bowersock '57, associate dean of the Faculty for undergraduate education, recently told the University they will soon soon depart, coincidentally, for the same place--the Institute for Advanced Studies in Princeton, N.J.

Frank B. Freidel, Warren Professor of American History, said this week he is not leaving, per se, but retiring a year early to accept a newly created post with a total annual endowment of $100,000 at the University Washington in Seattle.

The three most recent announcement brought the total number of tenured Faculty who have announced their departures this year to four. Yosef H. Yerushalmi, chairman of the Near Eastern Languages and Civilization Department, said in February that he will leave Harvard this summer for Columbia University.

Despite the flurry of farewells administrators don't seem overly concerned that it marks the beginning of a trend.

Dean Rosovsky said yesterday the average number of Faculty departures amounts to fewer than one every year. He added that because the number has been far below average for the past four or five years, the recent announcements balance things out.

"You've got to remember that we bring in a lot of people," Rosovsky said, adding, "There's got to be movement the other way as well."

"But these are three serious departures," he said.

Walzer said Thursday he "can't imagine" how the recent decisions to leave could constitute a trend.

He said he decided to join the Institute's school of social science because "it offers a kind of freedom which is both exiting and a little scary."

At the Institute--whose members have no teaching responsibilities--Walzer said he plans to work on several books, an activity "which would be much more difficult" if he remained at Harvard.

Bowersock--who is also professor of Greek and Latin--said this week he plans to finish a book on Rome and the Arabs when he goes to Princeton in the fall.

Both Walzer and Bowersock--whom Rosovsky described as "two of our best teachers"--did say, however, that they expect to teach occasionally at Princeton.

Freidel, on the other hand, will be teaching in Seattle as the Bullitt Professor of American history. Like the others, Freidel said he is not leaving out of "dissatisfaction with Harvard"; but he said, he does not want to go on mandatory half-time when he reaches the age of 65.

"I want to teach, and I want a full salary," Friedel said.