Defense lawyers for three Cuban exiles charged in connection with the 1976 bombing assassination of former Chilean ambassador Orlando Letelier have asked a federal judge here to suppress key evidence the government seized last March at the Union City, N.J., office of one of the defendants.

The evidence, materials the government contends are related to the construction of the bomb that blew up Letelier's car as he drove to work here on the morning of Sept. 21, 1976, was seized from the former office of Alvin Ross Diaz, a leader of the Cuban Nationalist Movement (CNM) charged with the slaying of Letelier and Letelier's colleague, Ronni K. Moffitt.

Ross and two other CNM leaders, Guillermo Novo Sampol and his brother, Ignacio Novo Sampol, go on trial today in U.S. District Court.

The key piece of evidence seized from Ross' office by FBI agents is a sales slip from Grand Central Radio in New York City that allegedly has Ross' name on it.The sales receipt was for the purchase of electronic equipment that the government contends was used to help detonate the bomb by remote control.

The defense lawyers, Paul Goldberger, Lawrence Dubin and Oscar Suarez, all of New York City, contend that the government prosecutors told them only last week of the sales slip, although the seizure of alleged bomb-making materials from the office has been known for several months.

In a separate legal brief, the government's prosecutors, Assistant U.S. Attorneys Eugene M. Propper, E. Lawrence Barcella Jr. and Dianne H. Kelly, have contended that defence lawyers were aware last June of the materials seized from the Union City office, but simply did not realize their significance until last week. That, the prosecutors maintained, "is their burden, not the government's."

U.S. District Judge Barrington D. Parker probably will hear arguments on the defense motion to suppress the evidence before government and defense lawyers start today to pick a jury to hear the case, which may last six weeks. Because of the nature of the charges in the case and death threats that have been made against Parker and Propper, security is extraordinarily tight at the federal courthouse.

Parker issued rules for the jury yesterday, saying that the sequestered jurors will be allowed to have two alcoholic drinks before dinner each day, but only if the jury is not meeting after dinner.

But Parker said that U.S. marshals will monitor any incoming mail to the jurors, as well as their telephone conversations and weekend visits with relatives. The judge said that jurors could read newspapers and magazines as long as articles about the trial are snipped out first, but that they could not listen to any television or radio news broadcasts.