A key government witness in the trial of three men charged with conspiring last year to assassinate prominent Iranian exile Ali Akbar Tabatabai at his Bethesda home testified yesterday that he and the alleged triggerman went to the victim's house the night before he was killed to carry out the shooting but that Tabatabai was not home.

As a result, an elaborate, execution-style slaying of Tabatabai took place the following morning, on July 22, 1980, in which an employe of the Iranian Interests Section of the Algerian Embassy posed as a mailman and lured Tabatabai to the door of his residence, where he was fatally shot.

The witness, Al Fletcher Hunter, also known as Abu Bakr Zaid Sharriff, is an unindicted coconspirator in the Tabatabai slaying. He agreed to cooperate with government investigators in their probe of the complex assassination plot in exchange for an agreement that he would not be prosecuted if he told the truth.

Hunter testified in D.C. Superior Court yesterday that after the shooting, he drove the alleged triggerman, Daoud Salahuddin, also known as David Belfield, a security guard in the interests section, to Montreal. Belfield is believed to have since fled to Iran.

Tabatabai was an outspoken critic of Iranian leader Ayatollah Ruhollah Khomeini. Tabatabai's assassination was labeled a conspiracy in which cash payments were involved, according to an indictment last July that charged Salahuddin and three others.

The three men accused of conspiring with Salahuddin to kill the exiled diplomat are Ahmed Rauf, also known as Horace Butler, who allegedly helped Salahuddin obtain a U.S. Postal Service truck as part of a ploy to pose as a mailman; Ali Abdul-Mani, also known as Lee Curtis Manning, who allegedly rented the vehicle used for the escape; and William Caffee Jr., also known as Kalid, who allegedly wiped the car clean of fingerprints and abandoned it in the District of Columbia. Rauf is also accused of disposing of the murder weapon, a 9mm pistol.

Hunter testified that he and Salahuddin went to Tabatabai's house, at 9313 Friars Rd., on July 21 and Hunter was told to go to the door and shoot Tabatabai.

Hunter said in response to questions posed by Assistant U.S. Attorney Harold L. Cushenberry that he carried the handgun and knocked on Tabatabai's door. When no one answered, they left. There was no indication whether the men on trial were involved in the July 21 visit to Tabatabai's residence.

Defense attorney Thomas Abbenante, who represents Rauf, said after Hunter's testimony that his admissions would help the defense case "because it shows what kind of witness the government has testifying for it."

Hunter testified that Salahuddin told him before the shooting that they would have an "assignment," apparently referring to the Tabatabai shooting. Hunter described the plot involving the use of the postal vehicle and Salahuddin's changing of clothes to disguise himself as a postman.

He said that after the shooting they went to Butler's apartment where Salahuddin changed out of his postal uniform. They made a call about airline reservations, then began driving north in a blue Toyota.

In Montreal, Hunter said he was told by Salahuddin to pay $300 to Tyrone Frazier, a postman who allowed his jeep to be used in the incident. Frazier is also expected to be a key government witness.

Government prosecutors charged on Thursday that a partial payoff for the shooting was arranged by an Iranian student who worked unofficially for the interests section. While in Montreal, Hunter said Salahuddin instructed him to call the student, Mehdi Safiri, in order to collect a $2,000 payment.

The trial continues before Judge Fred Ugast today.