Three unidentified men broke into the Bethesda home of Jamaican Ambassador Keith Johnson yesterday afternoon, beat and tied up a domestic employe, ransacked the bedrooms and escaped in a car owned by the family, Montgomery County police said.

The employe was the only person in the house when the robbery occurred, police said. The ambassador was in Jamaica yesterday, police said. An embassy spokesman identified the employe as Owen Hayden, a Jamaican who is caretaker of the house.

Police said a neighbor walking her dog about 12:40 p.m. heard the sound of moaning coming from the ambassador's garage and discovered Hayden tied up with rope and lying on the garage floor. He had been beaten on the head and face, police said, and he was treated for cuts and bruises at Suburban Hospital.

Montgomery County Detective Robert F. Renne said there were no signs of forced entry into the house, and it was not clear how the three unidentified men gained entry. Renne said that until embassy employes complete a check of the house, it will not be known whether anything besides the ambassador's car, which has diplomatic plates, was taken.

He said police do not know whether the three robbers were armed.

A county police spokesman said the FBI, Secret Service and county police will conduct a joint investigation.