By ROMAN CZAJKOWSKY

WASHINGTON--Two Cuban exiles were found guilty yesterday of the murder of former Chilean diplomat Orlando Letelier. A third Cuban was convicted of covering up the crime.

Friends and relatives of the guilty-all members of the Union City-based Cuban Nationalist Movement (CNM)-began weeping uncontrollably as the jury foreman announced the verdict against brothers Guillermo and Ignacio Novo and Alvin Ross.

Guillermo Novo and Ross were found guilty of conspiracy to murder a foreign official, murder of a foreign official, two counts of first degree murder and murder by use of explosives. Each count carries a maximum. sentence of life imprisonment.

The U.S. District Court jury also convicted Guillermo Novo on two counts of lying to the grand jury that investigated the Letelier case. Each count carries up to five years in jail.

Ignacio Novo was found guilty on two counts of lying to a grand jury and covering up the crime. He faces a total of 13 years imprisonment.

Letelier was killed when a remote control bomb exploded under the car he was driving to work on Washington's Embassy Row Sept. 21, 1976.

The three Cubans stood pale and motionless as the verdicts were read.

Judge Barrington Parker, who presided over the 5 1/2-week trial, immediately ordered them held without bond pending sentencing, denying a defense motion that Ignacio Novo be allowed to remain free on $25,000 bond.

The Novos and Ross stood still for several minutes after the verdict was read, then embraced each other and their defense lawyers - Lawrence Dubin, Paul Goldberger and Oscar Suarez.

As they were led out of the courtroom surrounded by a tight cordon of U.S. Marshals, Ignacio Novo, then Ross, shouted "Viva Cuba!" - "Long live Cuba!"

The all black, seven-women five-man jury reached its verdict after nearly eight hours of deliberations that began at 3:30 p.m. Tuesday, and after listening to 21 days of testimony and arguments.

The end came swiftly. Only minutes after the court clerk announced that the jury was returning, nearly two dozen U.S. Marshals took up position in the courtroom, forming a living wall that separated spectators from the defendants and jury. Security measures were conspicuous throughout the trial.

Then, just before the jury entered, Guillermo Novo whispered to relatives in Spanish: "It's certain that they cut our throats."

Despite the verdicts yesterday, the book on the Letelier case remains open.

The Novos and Ross were only three of eight persons named in a federal grand jury indictment handed down last August in connection with the case.

Missing from the courtroom were two other CNM members who are charged with murder - Virgilio Paz and Jose Dionisio Suarez. Both are targets of an extensive FBI manhunt.

The U.S. government also wants three former officials of DINA, Chile's secret police, to stand trial for the murder. The Chilean government has yet to act on a U.S. request to extradite them.

The prosecution has contended all along it was DINA that hatched the assassination plot and recruited the CNM, with which it allegedly had a longstanding relationship, to carry out the murder.

Letelier, who was foreign minister and ambassador to the U.S. under the government of Marxist President Salvador Allende, was a strident critic of Chile's military junta that took power after the 1973 coup in which Allende was killed. He was working for a leftist think-tank in Washington at the time of his death.

A young aide to Letelier, Ronni Moffitt, formerly of Passaic, was also killed in the blast. Her husband, Michael, escaped without serious injury.

The U.S. government has never directly blamed Chile's junta for the assassination, and junta strongman Augusto Pinochet has denied any prior knowledge of the plot.

Relatives of Letelier, including his widow, Isabel, and other anti-junta sympathizers have argued, however, that it is unlikely that Pinochet would have been unaware of the murder scheme, since he was the immediate superior and close friend of one of the three Chileans named in the case, former DINA chief Contreras Sepulveda,

Speculation is that yesterday's guilty verdicts will virtually finish the CNM - New Jersey's most notorious anti-Castro group - as a vital organization, even with verdicts in against fewer than half the persons named in the international murder plot.

FBI officials familiar with the movement noted yesterday that at its zenith the group had no more than 10-15 hardcore members. Now three of the group's key leaders face long jail sentences and two others are liable to be caught and tried.

The CNM is believed to have been responsible for a number of terrorist activities, including several bombing attempts in the New York area.

Most of the government's case against the Cubans hinged on the testimony of Michael Vernon Townley, the American expatriate and former DINA agent who testified he recruited CNM members for the murder on direct orders from DINA.

Townley, who has pleaded guilty to a count of murder conspiracy and is scheduled to be sentenced to a 3 to 10-year prison term, said he assembled, then placed the bomb under Letelier's car two days before it was detonated by remote control, either by Suarez or Paz.

None of the three Cubans convicted yesterday has been placed at the scene of the murder. But the government contended that Guillermo Novo and Ross discussed the murder plot with Townley at a meeting in Union City. Townley also testified he told Ignacio Novo-about the killing after it took place.

The cast of government witnesses also included three government informants - all convicts - who claimed Ross and Guillermo Novo admitted to them they role in the murder plot.

To the end, the defense strategy was to cast "reasonable doubt" on the truthfulness of the government witnesses and to show they collaborated with the prosecution only to have pending charges against them either reduced or thrown out.

Defense lawyers also suggested - but never proved - that Townley, who was expelled from Chile last year, may have killed Letelier on orders of the CIA. The agency gave Townley a conditional clearance for "operational capacity" use in the early 1970s, but said it had no contact with him after 1973.

Prosecutors Lawrence Barcella and Eugene Propper, however, labeled the CIA theory a "smokescreen," and said the CNM went along with the murder scheme in return for "political considerations" for the group by Chile's government, including recognition of the CNM as Cuba's government in exile.

Judge Parker yesterday gave defense lawyers, who said they would appeal the verdicts, until March 12 to prepare post-trial motions. No sentencing date was set.