Defense lawyers for three Cubans charged in connection with the 1976 bombing assassination of former Chilean ambassador Orlando Letelier began their effort yesterday to sow seeds of doubt about the prosecution's case.

On the one hand, the defense tried to show that someone other than the Chilean secret police, DINA, might have been motivated to kill Letelier. The government has contended that DINA ordered Letelier's assassination because of his strident criticism of the current Chilean regime of military dictator Augusto Pinochet.

At the same time, the defense attempted to cast aspersions on the testimony of the government's key witness, Michael Townley, an American -born DINA agent who has confessed planting the bomb in Letelier's car that later exploded along Washington's embassy row on the morning of Sept. 21, 1976.

The defense's first witness, Letelier's widow, Isbel, told the U.S. District Court jury that she and her husband had noticed that their mail in Washington had been opened before they received it.

In one instance, she said that they received information about a Maryland driver's license in an envelope postmarked in the District of Columbia and that a Metro schedule arrived in an envelope postmarked in Rockville.

At first, Mrs. Letelier testified, she thought that DINA was opening their mail, but then decided that it also could be the FBI or the CIA.

The defense has contended that Townley actually was a double agent, working for both DINA and the CIA, and that the CIA ordered Letelier's slaying.

Mrs. Letelier, who earlier had testified for the prosecution, said that she was afraid "of being followed by DINA" while she was in the U.S. But she said that she did not think that "DINA alone" could have performed the mail-opening operation and that an American agency might have been involved.

Townley testified earlier in the trial of the three Cubans that he had only four contacts in the early 1970s with the CIA in the U.S. and that he never worked for the agency. According to a CIA affidavit, one arm of the agency once sought clearance for Townley's use in "operational capacity."

Yesterday, Edward W. Cannell III, a marine stationed at the U.S. Embassy in Santiago in the early 1970s, testified that he saw Townley attend social gatherings on six to eight occasions at the Marine House in the Chilean capital.

Cannell said that the weekly "happy hours" also were frequented by personnel who worked in the U.S. embassy's top secret POL-R division, an acronym that he said stood for "political research," and allegedly was the CIA's office in Santiago.

Cannell conceded under cross-examinaton that he had never seen Townley actually talking to anyone from the POL-R division at the happy hours, nor had he seen Townley enter the POL-R offices at the embassy.

Another defense witness, FBI agent Stuart W. Case, testified that Townley said that the "leg" wires on the blasting cap he used to make the bomb were yellow and purple. A government witness, Jose Barral, said he had turned over to the FBI an identical blasting cap to the one that apparently was used to make the bomb and its leg wires were yellow and orange.

Under cross-examination, Case said that blastng caps made by the same company often have different colored leg wires.

Earlier yesterday, Judge Barrington D. Parker denied defense lawyers' motions to acquit defendants Guillermo Novo Sampol and Alvin Ross Diaz, who are charged with the Letelier killing.

Parker denied a similar motion to acquit the third defendant, Ignacio Novo Sampol, on charges that he lied to a grand jury investigating the Letelier killing, but reserved judgment on a motion to dismiss a charge that Ignacio Novo failed to tell U.S. authorities about the Letelier murder.