The Boston Celtics sat down on the floor after a light workout at the Christian Brothers Academy in suburban Lexington today and had a 45-minute rap session on how to beat the Philadelphia 76ers.

"Just like those debates they have over at Harvard," said Dave Cowens, the taxi cab philosopher and center of the defending National Basketball Association champions. "An open forum."

The Celts and Sixers meet in the third game of the best-of-seven Eastern Conference semifinal playoff series Friday night at Boston Garden. The Garden will also host the fourth game Sunday afternoon (1:30 p.m., WTOP-TV-9).

"These two are really important," said Celtic captain John Havlicek after today's tune-up and group therapy session. "We don't want to give up the home-court advantage we earned in Philadelphia."

The Celtics stunned in 76ers in the opener in Philadelphia Sunday, erasing a nine-point deficit in the last 11 minutes in a manner reminiscent of the old Big Green palyoff machines of the past. They won, 113-111, on 20-foot jumper by Jo Jo White at the buzzer.

The Sixers, who started the series with the home-court edge because they finished first in the Atlantic Division over the 82-game regular season, evened the series with a 113-101 victory Wednesday night at The Spectrum.

The primary topic of discussions in the Celtics' "let us reason together" meeting today were:

1. How to defend against Julius Erving, which is admittedly the kind of dilemma worthy of debate at Harvard presumbaly at the medical school;

2. How to get more movement into the offense, which was less than free-flowing Wednesday;

3. The importance of getting ahead early dictating the tempo of the game.

Cowens spent a day off last week getting a hack's license and driving a cab around Boston, kibbitzing with fares who for the most part did not recognize him or know what he does in unreal life. He suggested that stagnating on offense was the best way to play into the hands of the 76ers.

If the playoff were decided by a series of one-on-one game, Philadelphia would win the title hands down. As everyone knows, they are rich in talent, not to mention just plain rich, but have an ego problem: too many and too big. Sometimes the individuals work at cross purposes.

Therefore, the Celtics have concluded, the best way to beat the 76ers is to emphasize their old virtues - a fast-breaking offense, pressing defense, intensity, togetherness, and that special elan that has characterized Boston teams in Red Auerbach's long regime as coach, general manager, and now team president.

The Celts have to dazzle them with teamwork, coach Tommy Heinsohn stressed. Keep the 76ers on the move, force them play as a unit. The Celtics are confident Philadelphia cannot do that.

Similarly, Heinsohn feels that getting an early lead is important. The Celtics have not been a good come-from-behind team all season, and are convinced they must impose a pace and make the Sixers react.

hat is why the role of Sidney Wicks is so important. The 6-9, 225-pound forward has been the Celts' sixth man since Feb. 2, a role he had not played since his sophomore year at UCLA.

When Wicks comes in, Havlicek usually shifts form forward to guard, spelling either White or Charlie Scott. Wicks is counted upon to give the team a lift, a shot of adrenalin, with both rebounding and shooting. In short, he must make things happen, as Havlicek did for so many seasons in the Celtics' time honored "sixth man" theory.

As for what to do against Erving, that remains the great debate, Havlicek - who has play in 167 playoff games in 13 years, more than any other player in history - has the principle responsibility for guarding the incomparable Dr. J. But the fact that Havilcek is one of the great defensive players of all time is not enough.

Erivng scored 66 points in the first two games, hitting 26 of 53 shots from the floor. Except, of course, he is seldom on the floor. More often he is in the rafters performing a flying circus beter than Monty Python.

Havlicek, the link to the great Celtic tradition of the past, has been scoring - 19 points the first game, 31 the second. But Erving has been to much for him to contain alone, and Heinsohn is sure the 76ers will key on the Celts' 37-year-old leader in game 3.

Should the Celtics have Havlicek play the Doctor straight up? Or should teammates be looking to help out and risk having Erving levitate and hit the open man, as he is so capable of doing?Opinion was mixed, and it is likely Boston will try some of each.

In other action, the Denver Nuggests will play the Portland Trial Blazers in the second game of their conference semifinal series. The contest will be televised in Washington Firday at 11:30 p.m. over WTOP-TV-9. Portlaland leads the series 1-0