The Bulgarian national crew, making its first appearance at the Henley Royal Regatta, swept to three victories in the final day of competition yesterday including a three-quarter length triumph over defending champion University of Washington in the premier event, the Grand Challenge Cup.

The Washington loss, followed by the defeat of Yale University's freshmen in the final of the Ladies' Challenge Plate of men's heavyweight eights, meant that for the first time since 1963 there were no American winners at this mecca of rowing.

The Potomac Boat Club eight from Washington. D.C., with three former Olympians and men aged up to 43 on board, were losers by one length to Leander Club of England in a semifinal in the Thames Challenge Cup.

Leander grabbed the lead at the start, Potomac had a bit of steering trouble. Potomac rowed about two strokes per minute slower than Leander, which lost in the final to the London Rowing Club.

"It was about three-quarters of a length at the finish," said Potomac Club member Bill Middendorf, who rode in the umpire's launch during the race. "What we needed was another quarter mile."

"I was a little upset that we let them move out there between the Barrier (2,089 feet from the start) and Fawley (3,435 feet)," said J. M. Forster, No. 2 man in the Potomac boat.

The Potomac crew had trained together for only two weeks before coming to England for last weekend's Nottingham Regatta and Henley. The crew rowed its first 2,000-meter race together at Nottingham.

"We've been getting appreciably better every time we've gone out," SAID No. 4 man Townsend Swayze.

Bowman Tony Charlton believes that Potomac's lack of on-the-spot coaching may have been crucial. Coach Ken Hoffman was called back to the United States after only one day in England because of a death in the family.

The Bulgarians, most of whom are soldiers with special six-month leaves for crew training, won the Grand, the Prince Philip Challenge Cup for coxed fours and the Steward's Challenge Cup, also a four-man event.

"We wanted to come to Henley because it's like a myth in rowing. We're happy we met the success," Bulgarian Coach Nikolai Zdravkov said through an interpreter.

Their Grand entry was unusual. The coxswain lay flat in the bow of the boat, his back to the eight-man crew, and concentrated on sterring, an innovation in European rowing over the past few years but not seen before at Henley.

Racing in a strong headwind on the River Thames, Washington's varisity crew took a three-foot lead at the start, but the stronger Bulgarians pulled away in mid-course and never looked in trouble.

"They just rowed us under," said Washington Coach Dick Erickson. "They had the power and experience, but we've got nothing to be ashamed of."

Yale's freshmen were surprise qualifiers for the Ladies' Plate final. The Yale crew rowed a superb semifinal defeating the Lady Margaret Boat Club of Cambridge, England, by one-half length.

Cheered on by former Yale President Kingman Brewster, now U.S. Ambassador in London, the Yale Frosh were prevented only by wayward steering from making a closer battle of the final. The Bulldogs lost by two-thirds of a length to Imperial College of London.

A Syracuse University freshmen junior varsity eight lost a bid for a place in the Ladies' Plate final, beaten by 1 1/4 lengths by Imperial College.