Bruce Davidson of Unionville, Pa., a member of the U.S. equestrian team, yesterday became the first rider to successfully defend the individual title in the World Three-Day Championship. He did it aboard Might Tango, a 7-year-old gray gelding.

A crowd estimated at 70,000 watched Davidson and mount finish with 93.4 penalty points. In 1974 Davidson won aboard Irsh Cap in Burghley, England.

The final day of stadium jumping was marred by a serious fall in which Caroline Treviranus of Berryville. Va., suffered a head injury. She remained unconscious with a concussion and was listed in serious condition at the University of Kentucky Medical Center last evening.

Canada won the team championship, with West Germany second and the Americans a disappointing third.

Davidson had expected to ride Irish Cap here, but two weeks ago the horse was injured and Davidson had to opt for Might Tango, a former racehorse, which most consider too young for such a grueling competition.

Following the speed and endurance tests on Saturday, Might Tango suffered from heat prostration and had to be immediately relieved with oxygen and ice. There was some speculation that the horse had received a for bidden substance, steroids or cortisone. The official veterinarian reported finding no such illegal drugs in a test taken before the final round.

John Watson, the only rider from Ireland, won the silver medal after finishing with 120.6 (low score wins) points on Cambridge Blue. The bronze medal went to Helmut Rethemeier of West Germany on Ladalco.

Treviranus, 24, received a head injury when her horse Comic Relief faltered going into a three-rail jump and threw her over its head.

Treviranus was rushed by military guard helicopter to the University of Kentucky Medical Center.

Treviranus was fifth in the overall individual standings going into the stadium jumping competition.

Treviranus is known for her courageous riding. In the previous world championships four years ago, she broke her collarbone in a fall on the cross country course, but remounted and completed the course over 10 more jumps after the injury.