Three workers at a lucrative out-call massage service were sentenced yesterday by a federal judge in Alexandria for engaging in a prostitution conspiracy in the Washington area.

District Judge Albert V. Bryan Jr. ordered Barry William Sheehan -- who bought the business from convicted prostitution ringleader Louis Michael Parrish two years ago -- to serve a total of 18 months in prison on five counts of conspiracy, the interstate transportation of women for immoral purposes and interstate travel to promote prostitution.

Bryan said he disagreed with a statement made by Sheehan's lawyer, James Clarke that Sheehan had violated the letter, but not the spirit, of federal antiprostitution laws.

Sheehan and two colleagues, Chloe Ann Dyer and Margolus Oxendine, were convicted in a two-day jury trial last month. Prosecutors said the out-call service, Dial-Us, used telephone banks in Alexandria and the District as a front for prostitution that earned some women employes as much as $2,000 a week.

Bryan yesterday sentenced Dyer to a suspended one-year prison term, fined her $500 and placed her on two years' probation. Bryan said he believed Dyer, who, according to testimony, was employed mainly to answer incoming telephone calls.

Oxendine, who was general manager of Dial-Us for about four weeks before the business was raided by the FBI and closed in January 1980, received a two-year suspended prison term and two years' probation.