A jury has convicted two former National Guardsmen of murdering two U.S. labor advisers and a Salvadoran land reform official, but prospects appeared bleak that the officers who reportedly planned and ordered the killings five years ago ever would come to trial.

In a rare display of frankness in a Salvadoran courtroom, the prosecutor told the jury that the trial was not only of the two defendants, but also of "the National Guard's death squad."

Assassination teams, many of them linked to the National Guard and the nation's two other military security forces, killed tens of thousands of real and suspected leftists between 1979 and 1983. Nevertheless, no military officer ever has come to trial for any of the murders.

A five-member jury found Santiago Gomez and Jose Valle guilty of aggravated homicide at about 11 last night at the end of the one-day trial. The judge must hand down a sentence within 15 days, and the two face a maximum penalty of 30 years in jail. El Salvador's constitution prohibits the death penalty.

The two defendants were corporals in the National Guard when they gunned down Michael Hammer of Potomac, Md., Mark Pearlman of Seattle, and Jose Rodolfo Viera, head of the Salvadoran land reform agency, on Jan. 3, 1981.

Hammer and Pearlman were employes of the American Institute for Free Labor Development, a branch of the AFL-CIO, and were advisers to the land reform program. The three were murdered as they dined at a restaurant at the Sheraton Hotel here.

The murders are the best documented single case of death squad activity that directly involved Salvadoran officers. The confessions of the two men convicted yesterday, and investigations by the labor organization, the FBI and the U.S. Justice Department, yielded strong evidence that National Guard Lt. Rodolfo Lopez Sibrian and Capt. Eduardo Avila were two of the masterminds of the crime.

Lopez Sibrian, for example, ordered Valle and Gomez to kill the three men just minutes before the slayings took place, according to official U.S. government accounts of the case. Avila, standing outside the hotel with Lopez Sibrian, gave Valle a .45-caliber automatic weapon and a khaki windbreaker to conceal it, according to the documents.

At the trial, the defense attorney cited these orders as justification for acquitting the two guardsmen. He read the jury the clauses in the armed forces' regulations that require enlisted men to follow any order from a superior immediately.

Despite the evidence implicating Lopez Sibrian and Avila, efforts to prosecute them have foundered. Critics have charged that the lack of legal action shows that the Salvadoran justice system is so intimidated by the armed forces that the officer corps is above the law.

In the trial yesterday, the prosecutor said that another prosecutor had withdrawn from the case after his father received a threatening phone call related to it.

The Salvadoran Supreme Court on Nov. 16, 1984, upheld a lower court ruling that dismissed the charges against Lopez Sibrian for lack of evidence. President Jose Napoleon Duarte ordered him expelled from the armed forces the next day, and he is believed to be outside the country.

The case against Avila remains open, as prosecutors may submit evidence against him until Nov. 11. But an appellate court threw out the last initiative to introduce new evidence against him.

The U.S. Embassy issued a statement today welcoming the convictions but adding that it still expected that "the intellectual authors and accomplices" of the crime would be prosecuted.

"We think there are a couple of arrows in our quiver," a U.S. official said. He declined to elaborate.