A federal court jury yesterday convicted three operators of a call-out massage service on conspiracy charges for their role in running a prosperous Washington-area prostitution ring employing women who made as much as $2,000 a week.

The prosecution in the two-day trial in Alexandria included testimony by a series of acknowledged prostitutes, some of whom said they had cooperated with the FBI while working at Dial Us, a call-out massage service with offices in Alexandria and the District.

The six-man, six-woman jury deliberated 3 1/2 hours before returning its verdicts against Barry William Sheehan, 37, co-owner of Dial Us; Sheehan's girlfriend, Chloe Ann Dyer, 38, and Margolus Oxendine, 47, general manager of the now-defunct operation.

Besides the conspiracy charge, Sheehan also was convicted on three of four alleged Mann Act violations involving the interstate transportation of women for immoral purposes, and a single count of interstate travel to promote prostitution.

None of the defendants showed any sign of emotion as the verdicts were read. U.S. District Judge Albert V. Bryan Jr. set sentencing for Dec. 11 and allowed them to remain free on bond.

The conspiracy and interstate travel charges carry a maximum penalty of five years in prison or a $10,000 fine or both on each count. Mann Act violations are punishable by a five-year prison term and/or a $5,000 fine.

Dial Us was described in court testimony as one of 10 area call-out massage services purchased three years ago by Sheehan and a partner from convicted prostitution ringleader Louis Michael Parrish. Law enforcement officials at the time branded the Parrish operation a $1 million-a-year sex empire.

Sheehan's massage business was shut down in January 1980 by FBI raids at his offices at 1928 Duke St., Alexandria, and 310 Sixth St. SE in the District. Although Sheehan, now of Orlando, Fla., testified he was unaware of any prostitution and sought to run a legitimate massage service, prosecutors contended the businesses were fronts for highly organized and lucrative sex-for-money activities.

Dial Us kept careful financial records and ledgers, furnished paging devices for its female employes and often included "tips" or "grats"-- money paid for extra sexual services -- on credit card slips, according to evidence in the trial.

Sheehan acknowledged on the witness stand he was attracted to the massage business by the prospect of large profits. Asked by his attorney, James Clarke, why massage prices charged by Dial Us were so high, Sheehan said: "Greed."

During later questioning about his ties with Parrish, U.S. prosecutor Leonie M. Brinkema demanded of Sheehan how he knew in advance the massage business could be so lucrative. "Arithmetic," Sheehan said.

Although the defense stressed that Dial Us required its employes to sign written contracts stating that prostitution would be grounds for dismissal, Sheehan said he was unable to name anyone who had been fired as a result. Bryan cautioned the jury to weigh carefully whether such contracts were bona fide or merely shams.

An alleged co-owner of the massage operation, Jerry David Evans, has been indicted by a federal grand jury and is scheduled to be tried in Alexandria next month. The FBI is still looking for a fifth defendant in the case, Barbara Lynn Dodson, an official said yesterday.