Three D.C. residents, including a man convicted last month of a string of armed robberies, pleaded guilty yesterday to the fatal shooting of a Northwest pawnshop employe who was killed, the government said, because arthritis prevented him from lying on the floor quickly enough during a robbery of the store.

The two men and one woman announced their pleas to D.C. Superior Court Judge Frederick H. Weisberg just before the trial was to begin in the June 1985 murder of 33-year-old Steven Reynolds, who worked at Weinstein's Pawnbrokers.

Jerry T. Martin, the man accused of shooting Reynolds in the back while customers watched, pleaded guilty to felony murder while armed, and 33-year-old Annie M. Robinson and 48-year-old Edward H. Turner pleaded guilty to second-degree murder while armed.

Martin, 26, also pleaded guilty to two other robberies, while Turner and Robinson pleaded guilty to one other armed robbery. Turner additionally pleaded guilty to robbery in connection with the pawnshop incident.

Before entering their pleas, the three defendants listened yesterday as prosecutor Wallace H. Kleindienst described how Robinson entered the store near the intersection of H Street and Massachusetts Avenue NW posing as a jewelry customer. Kleindienst said Turner and Martin soon followed, announced a holdup and ordered everyone in the store including about five customers to get down on the floor.