A federal jury here yesterday convicted three men of robbing a Georgetown bank after a key prosecution witness refused to testify at the trial.

The witness, Vincent Dorsey, a towtruck driver for the city's Department of Transportation, complained to police when he learned the three bank robbery suspects were freed on personal bond while they were awaiting trial.

After the suspects were convicted yesterday, U.S. District Judge Gerhard Gesell ordered them jailed without bond pending sentencing. Convicted of bank robbery were Lewis Blakeney of 1815 Q. St. SE and twins Stanley and Bradley Holmes of 4404 Quarles St. NE.

The three were accused of holding up the National Savings and Trust Co. branch at 2929 M St. NW last Nov. 14. As they fled from the area, Dorsey chased them in his tow truck, radioed the getaway car's license number to his Transportation Department dispatcher, but then lost the car in traffic. Police arrested the three suspects a short time later.

Gesell held Dorsey in contempt of court Tuesday after Assistant U.S. Attorney William Collins said that Dorsey had torn up his subpoena and would not testify. Dorsey, who was held briefly in the courthouse cellblock Tuesday, was released pending a further hearing before Gesell on Feb. 20.