Federal authorities charged three more persons yesterday with conspiring to murder former Chilean ambassador Orlando, and are now confident they can trace the assassination from its origins in Chile to the actual placing and detonation of a car bomb here.

The charged against the three men bring to six the number of persons identified by investigators so far as being part of the murder plot. One of the three men charged yesterday was arrested in New Jersey, and the other two are fugitives, authorities said.

Additional charges are expected to be filed in the case - including the possibility of criminal allegations against officials of the Chilean secret police force, known as DINA, investigators added.

The suspects charged yesterday by the FBI were identitied as:

Ignacio Novo Sampol, 39, a leader of the New Jersey-based Cuban Nationalist Movement. Novo, the brother of another suspect in the Letelier murder plot, was arrested early yesterday at the home of his brother-in-law in the Newark, N.J., area after a brief scuffle when he tried to escape.

Jose Dionisio Suarez Esquivel, 39, who also live in northern New Jersey. Suarez, also a CNM leader, spent nearly a year in D.C. jail during the Letelier probe when he refused to testify before a grand jury after being granted immunity from prosecution. He is now a fugitive, the FBI said.

Virgilio Paz Romero, 26, also a CNM member in northern New Jersey. His name had not surfaced before in the Letelier investigation, and his alleged role in the plot is unclear. He also is a fugitive.

All three men were charged with conspiring to murder Letelier. In addition, Novo was charged with lying to a grand jury.

Letelier, an outspoken critic of the Chilean military regime, was murdered when a bomb exploded under the front seat of his car as he drove to work at the Institute for Policy Studies. An IPS aide, Ronni K. Moffitt, also died in the blast as the car drove around Sheridan Circle NW on Sept, 21, 1976.

The extensive FBI investigation of the murders proceeded slowly for more than a year, but in the last two months has speeded up considerably. Yesterday's charges were the latest in a series of developments that makes it clear the U.S. authorities now feel the case will ultimately be brought to trial here.

The investigators have determined to their satisfaction that the murder was carried out by anti-Castro Cuban exiles in the United States acting at the request of the Chilean secret police force, sources have said in the past.

The alleged go-between from Chile to the Cubans in the United States, an American-born Chilean secret police agent named Michael Vernon Townley, is cooperating with the U.S. authorities in the case, according to informed sources.

Those sources said the cooperation is "at the specific request" of Chilean President Angusto Pinochet. "He (Townley) is a soldier. His orders are to cooperate, and that is consistent with the personal wishes of Gen. Pinochet," said one source familiar with the Letelier investigation.

Townley was expelled from Chile last month after he was identified as one of two DINA agents who investigators said used official Chilean passports to travel to the U.S. and meet with other suspects about a month before the murder. The other DINA agent is a Chilean national, and therefore could be brought into the United States only through extradition proceedings following the filing of formal criminal charges against him.

Townley, who has reportedly agreed to plead guilty to the one charge of conspiring to murder Letelier that has been filed against him, is being questioned only about the Letelier bombing, one source close to him said.

Although Townley reportedly has information about other terrorist activities in other countries, U.S. authorities are believed to feel his cooperation is so significant in the Letelier investigation that they will not force him to testify about any other such missions, the source said.

The fifth and sixth persons alleged to be centrally involved in the Letelier assassination conspiracy are Guillermo Novo Sampol and Alvin Ross Diaz. Both men are in federal custody on charges unrelated to the Letelier investigation, but have been identified by Letelier investigators as suspects.

Letelier was the Chilean ambassador to the U.S. from 1971 to 1973 and also served as foreign minister in the cabinet of Marxist Chilean President Salvador Allende. He was an outspoken critic of the military junta that seized power from Allende in a 1973 coup.