Because of a fortuitous set of circumstances, Harvard's basketball team should win its third game of the year tonight at Boston University.

Last year the Terriers had three of the finest players in their history: Randy Cross, the school's all-time leading scorer; Ken Leary, their all-time leader in assists; and 6-9 Dick Moreshead, one of the country's leading rebounders. They all graduated.

Their top returnee this season, high-scoring Wally Birchler, signed a contract with the Baltimore Orioles and is ineligible to play. Then the biggest player on the squad, 6-8 Dave Delaney, decided to spend more time on his studies and quit the team. A week ago the Terriers' best sophomore, Skip Mortimer, who was averaging 16 points a game, slipped a disc and will be out for the season.

This adds up to a lot of walling and gnashing of teeth on Commonwealth Avenue, as well as some relief for the Crimson at 8 p.m. tonight. The Terriers have already lost three of four games this year. They bowed to UMass, 65 to 49, beat American International, 83 to 55, lost to Dartmouth, 79 to 57, and were whipped by W.P.I., 76 to 60.

B.U. Has Height

B.U. has a height advantage over the Crimson, but that's all. Center John Osgood (6-7) is a solid ballplayer, averaging 10 points and 15 rebounds per game. Forwards Randy Robinson and Dave Walker, both 6-5 sophomores, are good rebounders though unspectacular shooters.

The Terriers' high scorer is guard Ken Ackerman, who is averaging 16.8 points per game, largely because of his excellent driving ability. The other guard, 5-9 "Tom Thumb" Zavorskas, is a good ball handler but not a high scorer.

Harvard Coach Floyd Wilson said yesterday he plans to have the Crimson use a full-court press much of the time tonight. He was greatly encouraged by the way the press worked against Williams; only unusually bad shooting cost Harvard a victory in that game.

Saturday's game showed that Harvard has plenty of depth in the backcourt, but at forward the Crimson has problems. John Scott played as if he were in a trance; Harvard might as well have been using just four players. Center Barry Williams has been playing great basketball recently, but he can't be expected to pull down 21 rebounds every night.

He needs help either from Scott or from 6-3 reserve Charlie McGonaugle. But McGonaugle has been sick this week and may not be able to play this evening.

Tonight's contest is the Crimson's next-to-last game before vacation. The quintet plays at Tufts Friday, then competes in the Motor City Tournament in Detroit and plays at Western Ontario over the holidays.