The trial of three Cuban exiles charged in connection with the 1976 bombing assassination here of former Chilean ambassador Orlando Letelier resumed yesterday, and unexpectedly assumed some of flavor of the old television show "To Tell the Truth."

The judge, jury, lawyers and spectators in U.S. District Court were left asking a simple question: Will the real Carlos P. Garcia please stand up?

The confusion started when Luis Vega, a Union City, N.Y., bilding superintendent, testified that in late 1977 he rented an office to a man he knew as Carlos J. Garcia. In late February and early March of last year, as Vega was cleaning out the abandoned office, he said he found some materials he thought might be used to make a bomb, just like he'd seen on TV.

So FBI agents came and showed Vega some pictures, and the building superintendent picked out apicture of the man he knew as Garcia.

The government contended, at least until yesterday, that the man Vega knew as Garcia was actually Alvin Ross Diaz, one of the Cuban eliles charged with murdering Letelier and his Institute for Policy Studies, colleague Ronni K. Moffitt, as they drove along Embassy Row on the morning of Sept. 21, 1979.

Moreover, the government alleged that the items Vega found in the office rented to "Garcia," including potassium permanganate, detonating cords and so-called electric matches, were left-over supplies from the materials used to construct the bomb that killed Letelier and Moffitt.

But as Vega finished his testimony for the prosecution yesterday, the jury was excused and defense lawyer Lawrence Debin called another man into the courtroom of Judge Barrington D. Parker.

Vega then testified that the second man was actually Carlos P. Garicia, the man who had paid the rent on the Union City office. Vega said the same thing for the benefit of the jury moments later, but by this time Parker had sent the man claiming to be the real Carlos P. Gracia outside the courttoom.

Parker ruled that the defense could not have Vega identify Garcia in court for the jury, but could call Garica later as a defense witness.

To confuse metters further, Ross has submitted an affidavit in the case saying that he rented the office in late 1977 and early 1978. After Vega's confusing testimony, sources close to the defense said that Garcia and Ross were business partners and long-time friends from their days in Cuba.

While the prosecutors in the case -- Assistant U.S. Attorneys Eugene M. Propper and E. Lawrence Barecella Jr. -- watched as their second witness fumble an identification, Ross and codenfendants Guillermo and Ignacio Novo Sampol smiled broadly at Garcia.

Earlier, another Cuban exile, Jose Barral, testified that he had given an electric blasting cap to Jose Dionisio Suarez Esquivel, a fugitive charged with the murders. But Barrol said that he had talked only with Suarez about the blasting cap request, not Ross, who had accompanied Suarez to Barral's New Jersey home. Barral said he did not know what the blasting cap was used for.

Barral, who seemed extremely reluctant to testify, said he gave Suarez the blasting cap about one to four days before he heard of the Letelier assassination.