Three US. senators returned from an eight-hour visit to the Cambodian capital Phnom Penh today and said the Vietnamese -supported Heng Samrin government there had agreed to consider their proposal to open truck routes from Thailand to increase deliveries of famine relief supplies to Cambodia.

The senators met for 90 minutes with Foreign Minister Hun Sen and were told Phnom Phen would have no problem providing security for the vehicles. The plan calls for trucks operated by UNICEF and the International Red Cross to move supplies across a "land bridge" from Thailand into Cambodia on Routes 5 and 6, parts of which are disputed by Cambodian guerrillas loyal to ousted Khmer Rouge leader Pol Pot.

Precisely how security would be provided was not discussed, the senators told a press conference. Diplomats here assumed it would involve some of the 150,000 to 200,000 Vietnamese troops believed to be deployed in Cambodia. A Vietnamese official in Bangkok had earlier told the senators his government would help with security if so requested by Phnom Penh.

James Sasser (D-tEnn.), Max Baucus, (D-Mont.), and John Danforth, (R-Mo.), were traveling as a special Senate delegation to study refugee problems. Earlier they had visited settlements of Cambodian refugees in Thailand.

They were the first U.S. officials to visit Phnom Penh since 1975. But Sen. Baucus stressed there was no discussion of U.S. recognition of the Heng Samrin government. The visit was for humanitarian purposes only, he said.

The delegation was told the proposal would be taken up by the Heng Samrin government's Central Committee, but no indication was given when a decision might be reached. Sen. Baucus expressed optimism that the plan would be approved. Sen. Danforth called on governments with influence in Phnom Penh to push for acceptance.

"There is absolutely no reason why hundreds of thousands of people should be condemn to death because some central committee doesn't act," he said

The official Phnom Penh radio meanwhile attacked relief efforts by the United States and Thailand and criticized proposals for overland convoys, casting doubt on whether the senators' plan would be approved.

The radio commentary harshly denounced an initial U.S. appropriation of $7 million for Cambodian relief supplies, blaming the current famine on a 1970-75 "U.S. war of aggression" against Cambodia.

"Mr. Carter thinks that with this sum of money and some crocodile tears he can shirk off U.S. responsibility," the radio said. "He is grossly mistaken."

In a sign of the Heng Samrin government's apparent desire to prevent relief from reaching its internal foes and thereby starve them into submission, the radio condemned Western insistence on nonpartisan aid as an attempt to prop up "the remnant Pol Pot troops and other reactionary factions."

The radio said proposals to ferry food supplies to Thailand "for direct distribution" by truck to Cambodians "are unacceptable." It said such measures would "violet the independence, sovereignty and security of our country" and that the Heng Samrin government is fully capable of receiving and distributing international aid" by itself with oversight by international observers.

Phnom Penh officials told the senators that 2.25 million people faced severe hunger and that 165,000 tons of rice were required in the next six months. Some of it would come from 1.8 million acres of land now under cultivation, the officials said.

UNICEF and the Red Cross began flying supplies into Phnom Penh in August, and later relief ships were sent to the port of Kompong Som. However, these two channels currently deliver only a fraction of the estimated 1,000 tons a day that Cambodia needs, and distribution now extends only 60 miles out of Phnom Penh.

People in Khmer Rouge zones of control, meanwhile, have received supplies sent directly across the border from Thailand.

Danforth said the senators had not attempted to secure safe passage for the relief convoy from the Khmer Rouge. But he said he understood the Pol Pot position to be that UNICEF and the Red Cross could deliver food to anyone, regardless of what zone they lived in.

However, analysts here noted that attempts by both sides to use food as a weapon had helped create the famine conditions now prevailing in many parts of Cambodia. The Heng Samrin government has tried to stop deliveries of food to the Khmer Rouge, who would be assumed to try to do the same against Heng Samrin if given the chance.

The senators landed at Phnom Penh Airport aboard a U.S. Air Force 707 jet at about 8:45 a.m. They said they were received "with courtesy and hospitality" and taken to a guesthouse in the city.

They toured the city for about 90 minutes and saw Soviet-supplied rice being loaded onto trucks. Besides meeting with Foreign Minister Hun Sen (President Heng Samrin was out of the city, they were told), they visited an orphanage and conferred with Red Cross and UNICEF representatives stationed in the city.

On Thursday they are scheduled to fly back to Washington, where they will report on their trip to President Carter.

In another development, a spokesman for Thailand's supreme military command said that about 100 Vietnamese soldiers crossed into Thailand yesterday from Cambodia but turned back when they were spotted by a Thai Air Force patrol. The spokesman said the troops, "definitely armed and in Vietnamese uniform," were seen near the village of Sanio Changan in an area where about 100,000 Cambodians are believed to be waiting to cross into Thailand.

Later, the supreme command said, one Thai marine was killed and seven others wounded in a clash with "foreign forces" near the Cambodian border.The forces were not identified. A member of the intruding force was captured in a separeate clash and was being interrogated, the command said.