Three U.S. senators, just returned from Cambodia and Thailand, said yesterday that the lives of tens of thousands of starving Cambodians hinge on the willingness of their government to allow trucks bearing food and other supplies to enter the country from Thailand.

But the senators, Max Baucus (D-Mont.), John Danforth (R-Mo.) and Jim Sasser (D-Tenn.) said the Vietnamese regime in Phnom Penh had thus far turned down the suggestion, an attitude that Danforth characterized as "insane."

"We cannot accept the possibility that a government, or an alleged government, is going to willfully assign hundreds of thousands of its own people to a needless death," he said.

The three senators met with President Carter at the White House yesterday to report on their meetings with Thai, Cambodian and Vietnamese officials in Bangkok and Phnom Penh. They also showed him photographic slides of what they found inside Cambodia and in refugee camps along the Thai-Cambodian border.

After the meeting, the president appeared before reporters to say that the attitude of the Cambodian government was "discouraging" but that the United States is prepared "to proceed expeditiously in every possible way to relieve the extant suffering."

The massive famine in Cambodia, where international relief agencies estimate that 2.25 million people face serious food shortages, has this week become the focus of official and political activity in Washington.

On Wednesday, Carter announced that the United States was committing $69 million over the next year to international efforts to relieve the famine and provide badly needed medical supplies in Cambodia. He made the announcement only moments after his rival for the Democratic presidential nomination, Sen. Edward M. Kennedy (D-Mass.), had sharply criticized the administration's response to the Cambodian famine.

The three senators who met with Carter yesterday said that Cambodian officials in Phnom Penh had told them they would "seriously consider" the proposal to establish a "land bridge" along two highways linking Thailand and Cambodia for the shipment by truck convoys of food and other supplies.

But in later statements, Cambodian Foreign Minister Hun Sen was unenthusiastic about the proposal and Radio Phnom Penh attacked it as "unacceptable."

The senators said they plan to meet next week with U.N. Secretary General Kurt Waldheim to discuss the proposal and take other steps to pressure the Cambodian government to reverse its position.

They also stressed that shipment of food by truck holds the only hope of providing sufficient relief to the Cambodian people, given the extent of the famine and the logistical problems involved in distributing it in the countryside.

"The fact is there is only one practical way to solve this problem, only one, and that is trucks," Danforth said. "Air transportation can't deliver the tonnage needed. They [the Cambodian government] have the life-and-death decision."

"Only the regime in Phnom Penh stands in the way," said Sasser, who estimated that trucks bearing relief supplies from Thailand could be rolling into Cambodia a few days after receiving permission from the Cambodian government. "If they say no, tens of thousands of people will die in the next 30, 60 or 90 days."

According to a written report the senators gave to Carter, international relief agencies estimate that about 30,000 tons of food and medical supplies a month are needed to relieve the famine. Currently, 12,000 tons can be brought in by sea and 300 by air each month, the report said.

"Over the past few days, we have witnessed a human tragedy of enormous and unfathomable proportions," the senators wrote. "Without a massive and prompt international relief effort, the situation will continue to deteriorate . . . The survival of the Khmer race is in jeopardy."

While calling for prompt action, the senators refused to be drawn into a discussion of Kennedy's criticism of the administration.

"If they [the administration] have been overly slow, I'd say the whole world has been overly slow" in responding to the famine, Sasser said.

The senators also warned that the $69 million the United States has thus far committed to the relief effort may have to be increased after the initial six months of aid.

The State Department, issuing a statement of concern over the Cambodians' decision, said, "We hope this initial reaction is not their final word." The statement added that the administration will continue to press its proposals.

In another development yesterday, it was announced that a U.N. conference on aid for Cambodia, suggested by France, will take place Nov. 5 at U.N. headquarters in New York.