Thirty-five members of Congress have petitioned President Carter for action to assure Chile's extradition of three military officers accused in the 1976 assassination of exile leader Orlando Letelier here.

In a letter presented Wednesday by Rep. Tom Harkin (D-Iowa), the congressmen said the Chilean military junta should be told that if extradition is rejected in a count decision expected this month, the U.S. government will take retaliatory actions including cutoff of private-bank lending to Chile.

Harkin initiated the petition despite pleas from the State and Justice department that he hold off action until the Chilean Supreme Court rules on the U.S. appeal of the earlier decision by its cheif justice denying extradition. A White House spokesman said yesterday a reply to the congressmen's letter is being prepared.

Harkin said he has gathered 55 cosponsors for a congressional resolution that would demand sanctions against Chile but that he withheld it when Assistant U.S. Attorney Eugene Propper, prosecutor in the Letelier case, and Ambassador to Chile George Landau assured him two months ago that the Chilean court would reach its decision in July.

But the decision did not come and Harkin noted that it now appears that the court ruling will fall during the congressional recess, precluding prompt legislative reaction.

Micheal Moffit of the Institute for Policy Studies, whose wife Ronnie was also slain in the car bombing that killed Letelier, said, "We feel we've been taken" by the U.S. response to Chile's refusal so far to extradite.

Both Harkin and Moffit said U.S. officials at first had assured them that only extradition of the three officers would be acceptable, but lately there have been indications that an open military trial in Chile would be acceptable.

The letter to Carter signed by both Democratic and Republican congressmen declared: "Not a single crime of the dictatorship has been punished by a Chilean military court -- after more than five years of tortures, executions and disappearances documented by the United Nations, Amnesty International and others. Only extradition can guarantee full public knowledge of the Chilean junta's responsibility for this and other crimes and prevent further terrorism abroad."

Harkin and Moffit are skeptical of the independence of the Chilean Supreme Court, whose cheif justice, Israel Borquez, rejected the U.S. case for extradition, which was based mainly on testimony of convicted American plotter Micheal Townley. A three-judge panel of the court is considering the U.S. appeal of Borquez's decision.

Any cutoff of private lending to Chile would be uprecedented in that no such action has ever been taken against a country with which the United States maintains relations.

The carter administration has ended aid to Chile because of its human rights record but the junta has replaced those public funds with extensive borrowing from private American banks.

Harkin and Sen. Edward Kennedy (D-Mass.) have proposed legislation requiring public disclosure of such private lending.