The Chilean government last night arrested and placed under military detention three Chileans indicted yesterday in Washington in the September 1976 bombing death of former Chilean foreign minister and diplomat Orlando Letelier.

The three were identified yesterday in the indictment as Gen. Juan Manuel Contreras Sepulveda former head of Chile's secret police (DINA) and two other DINA employes, operations director Pedro Espinoza Bravo and agent Armando Fernadez Larios.

The government statement announcing their arrests failed to say where they were being held or when they had been arrested.

The U.S. Embassy here had formally asked for the arrest and detention of the three yesterday afternoon touching off what is expected to be a complicated legal battle over their extradition.

The statement also stressed that the three are innocent until proven guilty and expressed the hope of Chilean President Augusto Pinochet that the case would be cleared up.

Contrearas is a close associate of Pinochet.

The statement, issued by Interior Minister Sergio Fernandez said there would be an extradition hearing before a Chilean court but no date for the hearing was given.

A U.S.Embassy note asking for the arrest was delivered by Charles Grover second deputy chief of mission with the concurrence of U.S. Ambassador Geroge W. Landau who was in northern Chile when the indictments were announced yesterday.

The detention of the three is the first step in extradition proceedings that probably will culminate ultimately with a hearing before the Chilean Supreme Court according to terms set forth in a 1900 treaty between Chile and the United States and later amended in 1935.

The legal battle over the extradition of the three will hinge on two seemingly contradictory provisions of the 1900 treaty, Chilean legal experts say.

According to the first provision, murder and "comprehending assassination" are extraditable offenses. But a second provision clearly sets forth that a "criminal shall not be surrendered if the of a political character or if he proves that the requisition for his surrender has, in fact, been made with a view to punish him for an offense of a political character."

The three suspects are expected to fight extradition.

Contreras has already retained a prominent Santiago lawyer. Sergio Miranda Carrington who once offered to defend Nazi war criminals at the Nuremberg trials at the end of the World War II.

Miranda could not be reached for comment but is expected by other lawyers in Chile to argue that the Letelier assassination was a political crime and is therefore not covered by the treaty.

However, one source pointed out yesterday that Contreras and the other two Chileans are also charged with the murder of Ronni Moffitt, the female colleague of Letelier who was riding in his car at the time it was blown up.

This source said a possible way around the treaty's prohibition against extradition for politiacal offense might be to argue that Moffitt's death could not be considered politically motivated and that the three Chileans should be extradicted to stand trial in the United States for her murder.

Another section of the 1900 treaty clearly states that "neither of the contacting parties shall be bound to deliver up its own citizens or subjects under the stipulations of this treaty." This section is interpreted here to mean that the Chilean Supreme Court could order extradition, but is not obliged to do so, even if the court decides there is sufficient evidence for the three Chileans to stand trial for the charges brought in the United States.

Another complication cited by legal sources here is that under Chilean laws conspiracy to commit a crime is not in itself a crime. The Supreme Court these sources said might well decide not to grant extradiction for that reason.

The 1935 treaty says that if the person whose extradiction is sought is a citizen of the country to which the request of a crime in another country when that country plans to ask for extradiction.

Even if the Chilean Supreme Court does not order the three Chileans to be extradicted to stand trial in the United States many observers believe the airing of the evidence against them in public will have a significant impact on Chilean public opinion about the alleged involvement by their government in the assassination of Letelier who served as the late Salvador Allende's ambassodor to Washington as well as his foreign and defense minister before Chile's 1973 coup.