Three employees of the Red Lobster Inn in Alexandria were seriously wounded Sunday night by a shotgun blast fired by one of two masked holdup men who then escaped with the day's cash receipts, Alexandria police reported.

Police said the men, wearing ski masks and armed with a knife and a shotgun, entered through the front door about 11:20 p.m. more than an hour after the restaurant at 555 S. Van Durn St. had closed. Police speculated that the door may have been unlocked.

After forcing Night Manager Michael McCool, 23, to open a safe, the men forced McCool and three other employees into a walk-in freezer at the rear of the restaurant, according to Police Lt. Otis Pettit.

The holdup men then shut the freezer door, but opened it a moment later and one of the men fired the shotgun into the freezer four times, Pettit said.

Only one employee, a woman whom Pettit would not identify, was uninjured. "She was in the back (of the freezer) and was apparently covered by the other three (employees). When the shooting started she fell down to the floor," Pettit said.

Admitted to Alexandria Hospital were McCool, suffering from a wound to the right hip, reported in serious condition: Joseph Arnoto, 31. the assistant manager, with wounds to his right arm and abdomen, reported in guarded condition, and Stewart Kerchner, 25, an employee of a Red Lobster Restaurant in Saugus, Mass., who was visiting McCool, with a wound to his left knee, in fair condition.

Last night Alexandria police skin-divers were searching the Potomac River off Memorial Bridge for the gun used in the shootings. They would not say what led them to believe the gun might be there.

Vincent Clark, supervisor for the Red Lobster Inn chain in the Washington area, said the restaurant was open for business as usual yesterday. The Red Lobster chain has 224 "family seafood restaurants" in 34 states.

Asked why the holdup men shot the employees after they had gotten the money, Pettit said, "We really don't know. It doesn't make a whole lot of sense."

The shootings at the Red Lobster recalled the slaying of four employees of a Roy Rogers Restaurant in Fairfax County in March, 1976. The man convicted of those slayings, James Leroy Breeden, 40, is serving five life terms in the Virginia Penitentiary in Richmond.

In the Roy Rogers incident, four men and woman were herded into a walk-in freezer, made it lie face down then shot.

One of the victims, the wife of one of the restaurant employees, survived even though she was shot in the head, and testified against Breeden at his trial.