President Carter, declaring that he would "not go back on my commitment" to U.S. allies, said last night that he will recommend about a 3 percent increase in defense spending to Congress in January.

In an interview with Barbara Walters broadcast on ABC, the president also gave unqualified support to former budget director Bert Lance, who is under investigation by a grand jury in Atlanta. The president's wife, Rosalynn, also suggested during the course of the interview that the president has already decided to seek reelection in 1980.

During the wide ranging interview, Carter also:

Confirmed that in the last fes weeks he has exchanged letters with Soviet President Leonid I. Brezhnev, warning him that the United States will not tolerate outside interference in Rran. He reiterated American support for the shah of Iran, but when asked that support might turn to "action" should Persian Gulf oil supplies be threatened, he replied, "I am not prepared to answer that question."

Described as "inconceivable" the possibility that congress would enact legislation giving him standby authority to impose wage and price controls, but said he would consider signing such legislation if the authority "could only be used in case of a threat to our nation's security ..."

Predicted that a new arms agreement with the Soviet Union will be reached "quite soon" and eventually will be approved by the Senate, and warned that a territorial dispute between Chile and Argentina is nearing the point of "conflict". He cited both matters as reasons for Secretary of State Cyrus R. Vance's decision to return from the Middle East today.

Said he foresees no changes in this Cabinet or senior White House staff during the reminder of his term. "I might want to improve some of them, but not move them," he said of his White House staff.

Said he cannot recall ever having lied to the Aerican people, even to protect national security interests.

Following the interview, an administration official said that no final decisions have been made on the size of next year's defense budget. Carter's comments did not entirely clarify the issue, the subject of intense debate within the adminstration and the Democratic Pary, but strongly suggested which way he is leaning.

The president reiterated his determination to fulfill his commitment to U.S. allies and replied "that's correct" when asked if he would recommend the increase of about 3 percent he pledged to those allies last year.

Carter offered "unqualified support" to Lance, whose tangled financial affairs are now the subject of a grand jury investigation and said he remains "proud of him."

"Bert is a fine, decent human being," the president said. "...I'm not trying to make a judgement on what the investigations will reveal. But I know Bert Lance, have confidence in him."

"He's just a good person," Mrs. Carter added.

Carter said it was much too early to announce his reelection plans, but his wife, who participated throughout the joint interview, volunteered the statement, "I think Jimmy is going to be president for a long time."

The president added that he is confident he could defeat Sen. Edward M. Kennedy (D-Mass.) or any other Democratic challenger to his renomination.

Carter also described as "absolutely false" suggestions that part of a multimillion-dollar loan he obtained from a Lance-controlled bank fo r his family peanut business had been funneled into his presidential campaign.

Both the president and Mrs. Carter said that there are fewer "misconceptions" about them and the adminstration than earlier. But Mrs. Carter cited her "frustration" with the lack of publicity over administration accomplishments, for example, a childhood immunization program she as supported. She also said she agrees with those who are urging Carter to appoint a woman to the Supreme Court when a vacancy occurs.

Asked if he had lived up to his prediction, made on the eve of his inauguration, that he would be a "great president," Carter replied, "I've not given up on it. But it's too early to make that judgement."