The United States formally asked Chile yesterday to extradite three former secret police officers charged with the murder two years ago today of Orlando Letelier, one of the most outspoken and effective critics of Chile's rightist military government.

The three are: Gen. Juan Manuel Contreras Sepulveda, former head of Chile's old secret police force known as DINA; Pedro Espinoza Bravo, who was DINA's director of operations when Letelier was murdered; and Armando Fernandez Larious, a DINA agent who alledgedly helped arrange the assassination.

The extradition request, along with a sachel containing more than 400 pages of evidence against the three Chileans as well as photographs and movie film, was delivered by U.S. Ambassador George W. Candau Chile's foreign minister, Hernan Cubillos, yesterday morning.

Shortly after receiving the formal diplomatic note and the evidence gathered by a federal grand jury in Washington, Cubillos met with reporters and said that the various documents would be sent to Chile's Supreme Court president, Israel Borquez, "within 24 hours."

Under the terms of a U.S.-Chilean extradition treaty ratified in 1902, the Supreme Court president, decides whether the evidence presented is sufficient to warrant a trial and, if so, in which country, the United States or Chile.

Borquez is expected to begin the extradition hearings sometime next week, when he will call the three men whose extradition is sought for questioning.

It is expected here that Borquez, who has shied away from any confrontations with the military government in his legal rulings, will take several months to reach his decision. The decision may then be appealed to a panel of five other Supreme Court members, who could spend another mouth or two reviewing the decision before it is final.

Once the initial hearing is held, all of the evidence gathered in the United States is expected to be made public here.

Some opponents of President Augusto Pinochet believe this could have a substantial impact on the strength and credibility of the five-year-old military regime that has ruled this country since 1973, when it overthrew the elected leftist government of Salvador Allende.

Letelier served as Allende's ambassador to the United States, and later held several Cabinet posts before the 1973 coup. Imprisoned by the military government for a year after the coup. Letelier had been living in Washington at the time a bomb attached to his car exploded near Sheridan Circle on Sept. 21, 1976.

The former ambassador was killed instantly and a passenger in his car, Ronni Moffitt, a co-worker with Letelier at the Institute for Policy Studies, died minutes later.

The three Chileans, along with four Cuban exiles who alledgedly carried out the assassination, were charged with both deaths last Aug. 1 after an exhaustive 22-month investigation by the U.S. Attorney's office in Washington.

Under the terms of the extradition treaty, Borquez has four choices, according to Alfredo Etcheberry, the lawyer who will represent the United States in the legal proceedings. The Supreme Court president could find that the evidence is not sufficient to warrant charged under Chilean law, in which case he would order that no trial be held.

If Borquez finds that the evidence is sufficient, he could decide that the Letelier assassination was a political crime, which would also bar a trial under terms of the treaty. The third choice would be to order a trial in Chile, probably on the grounds that extraditing a former head of the nation's intelligence service would impair Chile's national security. The fourth choice would be to order extradition.

Well-informed sources have indicated that the Carter administration would probably take strong diplomatic measures, including the possible recall of Ambassador Landau, should Borquez rule that the evidence is insufficient or if he ordered a closed trial before a Chilean military court. Even an open trial here could result in diplomatic retaliation if the verdict was considered unfounded or if the sentences were considered too lenient, these sources said.

Contreras, Espinoza and Fernandez Larios have all been under military arrest since early last month, when the United States requested that they be detained under terms of the treaty.

The Letelier investigation got its big break last April, when Chile expelled Michael V. Townley, an American citizen who was a DINA agent and who has admitted his part in helping to plan and carry out the assassination.

Townley began cooperating with U.S. authorities almost immediately after he arrived in Washington in the custody of FBI agents. He provided much of the evidence and many of the leads that led to charges against the three Chileans and five Cubans, two of whom are still fugitives. One of the Cubans is charged not with murder, but with failing to tell law enforcement authorities about a crime after it occurred.

The charges, especially against Contreras, have been extremely embarrassing for Pinochet, who is said to have had a close personal and working relationship with the former DINA head - a friendship Pinochet has since said was a fiction concocted by his opponents.

Pinochet has also denied any knowledge of the plans to kill Letelier. He has said that he intends to allow Borquez to make the extradition decision without any interference on the part of the government.