Florence Eyssalenne, a 22-year-old paralegal found slain in her Capitol Hill apartment Sunday morning, is the third employe of one Washington law firm to be killed since 1981.

Eyssalenne, who joined the firm of Spiegel & McDiarmid shortly after graduating from Harvard in 1984, was found with her throat cut in the bedroom of her locked apartment at 610 Third St. SE, D.C. police said.

A police source said Eyssalene apparently had also been beaten.

It is the third time in four years that a young, female legal assistant at the firm has been killed. In June 1981 Catherine T. Schilling, 21, was found dead of multiple gunshot wounds on a bank of Rock Creek under the Whitehurst Freeway.

Less than a year later, in May 1982, Helen Reed, 24, was found stabbed to death in front of the Mount Rainier apartment building where she lived.

Sgt. Bob Law, spokesman for Prince George's County police, said last night that investigators determined that Reed's slaying involved a robbery, but that no arrests have been made in the case. "To my knowledge, there's no connection between the murders and the place they worked for," Law said.

Thomas Trauger, a partner of Spiegel & McDiarmid, said Eyssalenne's death has shaken and saddened the firm's employes. It is even more disturbing, he said, for those who remember the two previous slayings.

"It is traumatic, depressing, shocking," said Trauger, one of 12 partners in the firm, at 1350 New York Ave. NW.. "We don't have any information that would suggest any connection among the three except that they all worked for the firm . . . . I think, for those of us who've been there for the whole period since 1981 , it's on our minds. These aren't things we forget."

Eyssalenne, a Brooklyn native, was described by friends in the student newspaper the Harvard Crimson as a quiet, friendly woman who studied French literature and served as a dorm crew supervisor.

Members of her family, reached at home last night, declined to comment.

The New York Post reported that Eyssalenne had an older brother, Bernard, and a younger sister, Monique, who attends Catholic school in Manhattan. The family, according to the New York Post report, has lived in the Prospect Park South section of Brooklyn for at least 10 years.

Trauger said the three women who have been slain, all recent college graduates, were liked and respected by partners and others in the firm, which has 80 employes. "In each case, they were bright and talented and ambitious, just the sort of people you would look for for that job."

Schilling, a Georgetown University student, had worked for the firm for about eight months when her nude body was found partially hidden in the underbrush on the east bank of Rock Creek about 300 yards from the firm's office, then located in the Watergate Building at 2600 Virginia Ave. NW. Coworkers, who began searching for her when she did not report to work, discovered the body near a path they said Schilling often used as a shortcut to her home at 3613 N St. NW.

Donald E. Gates was convicted in September 1982 of first-degree murder and rape in Schilling's death and is serving a life sentence.

Reed, a Howard University student, was found after police received reports of a woman screaming in the vicinity of 4107 Russell Ave., Mount Rainier, according to Law of the Prince George's police.