Leon Grant, a 30-year-old merchant who agreed to help police foil a planned holdup, was shot and killed yesterday outside the Riggs National Bank branch at 14th Street and Park Road NW while at least a dozen District police had him under surveillance.

Police arrested three men on the scene and charged them last night with homicide. Assistant Police Chief Bernard J. Crook said that "we're going to make a complete study in depth of this entire operation." Crook added that "we are going to do so so that we can evaluate this case and see if proper judgement was used, and if corrective acts are warranted, we will take [them]."

Grant, who had been outfitted with soft body armor provided by police, was hit once in the throat without warning, according to police. Bleeding heavily from his wound, he ran from the parking lot on Park Road where the shooting occurred, to the corner of 14th Street.

Rushed in a police car to Washington Hospital Center, Grant died at the hospital 45 minutes after the 3:13 p.m. shooting. He left a wife and two children.

Police gave the following account of the shooting:

Grant came to police at 4th District headquarters Wednesday, saying an attempt would be made to rob him as he left the Riggs branch - as he did every Friday - with the weekly cash payroll for his employes. Grant owned the firm of Grant & Green, 7323 Georgia Ave. NW, a flooring and refinishing firm that specialized in hardwood flooring.

Police did not specify how Grant knew of the threat, but he apparently gave them enough information for them to write a complaint. Crook said the information supplied by Grant was not strong enough to allow them to make an arrest before the robbery.

Crook said police had "unconfirmed information from a source from which we had never got information before, not enough for a warrant based on reliable information: we couldn't use a decoy because they apparently knew Mr. Grant."

On Thursday, police conducted a dry run of the operation, with Grant going to the bank, coming out of the parking lot where his Jeep was parked, "working out the plans on who was going to do what, guarding against all contingencies, with other police cars in the area to cut off escape routes and for the purpose of observation," Crook said.

Yesterday Grant went to the bank in midafternoon to withdraw $2,500 in cash, which he put into a beige imitation leather bag. He walked out of the bank, down Park Road toward the parking lot where his Jeep was parked, observed by six plainclothes police officers. At least six more police officers were in the immediate vicinity in patrol cars, police said.

As Grant neared his Jeep, Sgt. Benny Preston saw three men running toward Grant. Preston, in plainclothes, was 20 feet away, police said, when one of the three holdup men, without speaking or giving any warning, shot Grant in the throat seconds after he had seated himself in the driver's seat. The gunman took the money and started to run away, detectives said.

While the fatally wounded Grant fled down Park Road, one police officer fired two shots at the fleeing bandits and another officer fired once. The bandit carrying the bag dropped to the ground, the money bag on one side, his gun on the other. A second bandit continued to flee and ran into the arms of police and was arrested. The third suspect was captured by police nearby on Hiatt Place, where a getaway car had been parked, detectives reported.

The three men arrested and were identified by police as Leonard J. Anderson, 23, of 51 T St. NE; Melvin Steuart, 23, of 1132 7th St. NE, and Arthur Swarn, 26, of 863 21st St. NE.

"During the course of a year," Crook said, "we get many, many tips like this, the majority of which don't pan out. Each case is different. I'm not ready to say right now what we could have done or could not have done to have prevented this."

"Who could have anticipated that we were dealing with a cold-blooded murder?" another police officer asked.

Two neighbors described Grant, who live at 4016 Rickover Rd., Silver Spring, as "a very pleasant man" who worked hard at running his business.

Thomas Patton, who lives across the street from Grant at 4019 Rickover Rd., said Grant was a "good craftsman" who had refinished some wood walls in the Patton home and had done "just a superior job."

Patton also said Grant was "a knockout dresser - good looking suits, nice shirts, the whole works."

Patton and Margaret Smith, who lives next door to Grant, said Grant and his wife Ruth had broken up within the last two or three weeks. They said Ruth had moved with the couple's two daughters, Terri, 10, and Kelley, 2, to a town house in Reston while Grant stayed behind in the $100,000 contemporary brick and frame house they had lived in for 12 to 18 months.

Both Patton and Smith said they were flabbergasted at news of the killings and had no idea why it occurred.

Gran't wife arrived at the hospital at 6 p.m.