In the summer of 1974, on the palatial Burghley estate outside of London, the United States gave Great Britain its biggest shock since the American Revolution.

That was when Pennsylvania's Bruce Davidson came out of nowhere and became the first American to win the World Equestrian Championship - an event long dominated by the British. His surprise victory began a string of U.S! victories in international equestrian competitions - culminating in 1976 with Olympic gold medals in team and individual categories.

Yesterday, in hopes of solidifying its newly found position of supremacy, the U.S. equestrian team held the third in a series of five qualifying tryouts for the 1978 World Championships in Lexington, Ky!, scheduled Sept. 14-17.

By the end of this humid day on Walnut Farm, not far from the West Virginia border, Michael Plumb had ridden his horse, Better And Better, to a second straight qualifying victory with 46.8 points. Mary Anne Tauskey, on Marcus Aurelius, finished second with 52 and also rode The Sheik to third place picking up 55.8 points.

Derek diGrazia, a Georgetown graduate, took fourth on Thriller II with 63.2 points. The fifth spot was nailed down by Tad Coffin, gold medalist at Montreal, with 64.75 points on Karama Kazure.

Davidson withdrew from the competition early to rest his horse. But because of superior scores in the two prior qualifying rounds, he is still in contention for the final selection of 12 riders to represent the U.S. That process will begin in August, following the remaining competition in Boston and Pennsylvania in July. All riders, regardless of their finish yesterday, can give it another shot at each of those meetings.

The top finishers here, who along with Davidson hold an edge for making the USET, compiled the fewest number of penalty points in three categories: dressage, a series of finesse maneuvers; stadium or precision jumping, and cross country, a test of endurance and overall skill.

"Our main task as a team is to make the right selections and make sure we don't burn our horses out," said Plumb, the captain of the U.S. Olympic Team and veteran of five Olympics. "There's a lot of pressure on us since we've taken the title.The British and West Germans can't wait to get another shot at us."

Plumb, 38, who won a silver medal in the 1976 Olympics, began equestrian riding at the minimum allowable age of 18. While he is now considered older than the average rider, his 9-year old Better And Better is considered young for the sport. No horse under 5 years can compete - and many are still in the running at 15.