Three District men have been convicted of armed robbery in the holdup last summer of an armored truck in downtown Washington during which gunfire was exchanged with guards.

U.S. Attorney Joseph diGenova said yesterday that the verdict, reached late Friday night by a U.S. District Court jury, was the first here in which the 1984 federal Armed Career Criminal Act came into effect.

The act requires a mandatory minimum sentence of 15 years in prison with no probation or parole for offenders with three prior robbery or burglary convictions.

Convicted under the act was Horace Graydon Jr., 39, of 36 G St. SW. He was also convicted on four other counts -- armed robbery, armed robbery of an interstate shipment of money, use of a firearm during a federal crime of violence and assault with intent to kill while armed.

Anthony K. Henry, 25, of 1724 Potomac Ave. SE and Don A. Shaifer, 24, of 1407 Ames Place NE were each convicted on the two robbery charges.

The holdup took place Aug. 23 near Ninth and F streets NW and involved money being taken to a bank from McDonald's restaurants in Washington and Maryland. The shootout, which sent bystanders diving for cover but caused no injuries, occurred in front of the National Museum of American Art.

Judge John Lewis Smith Jr. ordered all three men held in jail until sentencing, which he indicated would take place next month.