Three U.S. senators are schedled to fly to Phnom Penh Wednesday morning to seek approval of a plan they have devised to truck internationally donated relief supplies directly from Thailand to famine-ravaged zones controlled by the Vietnamese-supported Heng Samrin government.

When they land in Phnom Penh, Sens. James Sasser (D-Tenn.), Max Baucus (D-Mont.) and John Danforth (R-Mo) will be the first official U.S. visitors to Cambodia since the Khmer Rouge captured Phnom Penh in 1975.

Analysts in Bangkok believed that the Vietnamese and their clients in Phnom Penh had approved the trip partly to gain increased international standing for the diplomatically isolated regime installed by the Vietnamese in January after toppling the Khmer Rouge government of Pol Pot. The United States has not had diplomatic relations with their government.

"The sole mission of our trip is humanitarian" Baucus said at a press conference here. "Our visit in no way . . . amounts to recognition of any form of government in Cambodia."

In Washington the State Department said the senators' visit should not be taken as indicating any move by Washington toward recognition of the Heng Samrin government.

Spokesman Hodding Carter said that for policy reasons, no senior U.S. officials will accompany the American legislators to Phnom Penh.

Assistant Secretary of State Richard Holbrooke, who went to Bangkok with the senators, will not travel with them to Phnom Penh.

"We have to be sure no mixed signal will come out of it," Carter said. "It seems safer not to have implications raised" about possible U.S. recognition.

Approval for the Americans' visit was received this morning from the Vietnamese embassy in Bangkok. Aides said the delegation would fly to Phnom Penh early tomorrow aboard a U.S. government jet that brought them to Bangkok.

The senators' visit comes at a time when Cambodia is faced with severe shortages of basic foodstuffs. Warfare and politically motivated bickering is slowing the delivery of international aid to the village level.

The three men are members of a special senatorial delegation on refugees. On Monday they traveled to the Cambodian border to talk with people who recently fled into Thailand in response to escalated fighting.

Today they detailed their proposals to open a "land bridge" into Cambodia. The trucks and supplies would be provided by agencies like the United Children's Fund and the International Committee of the Red Cross. The convoys would deliver the aid along routes 5 and 6, which pass close to areas in which guerrillas loyal to the ousted Khmer Rouge regime operate.

Methods of delivering aid that the Heng Samrin authorities have approved, by air to Phnom Penh and by sea to Kompong Som, have a capacity of less than half the 1,000 tons per day that Cambodia needs, Danforth said. "If permission were given tomorrow" for their proposals, he stated, "in a period of about three to five days trucks loaded with food would be moving from Thailand."

Vietnamese troops in Cambodia could be asked to provide security for the convoys, the senators said, a possibility that was discussed this morning in a meeting here with Vietnamese Minister of State for Foreign Affairs Nguyen Co Thach. Thach told them Vietnam would help with security if the Phnom Penh authorities so requested, the senators said.

The senators said they did not know who exactly might pose a security threat to the trucks, though one did mention the possibility of hijackings.

Indochina watchers in Bangkok believed that any threat would probably come from Khmer Rouge guerrillas who would not look kindly on hundreds of heavily laden trucks passing into the territory of their adversaries. Khmer Rouge enclaves are currently receiving some relief trucked in directly from Thailand. In Phnom Penh, Heng Samrin officials have repeatedly condemned those shipments and demanded that all Cambodian relief be channeled through their government.

The senators said they had not sought a guarantee of security from the Khmer Rouge, nor did they intend to.

When pressed for details on the expected threat to the convoys, U.S. Embassy sources said the questin was more one of clearance than of security. It is simply a practical problem, one source said.Aid "cannot be moved without the permission of the Vietnamese Army."

However, this explanation conflicted with the senators' statements, which implied the Vietnamese would be asked to provide some sort of escort force.

Sasser said that Thai Prime Minister Kriangak Chamanan had expressed "warm support" for the relief convoy idea. Thailand is anxious that food enter Cambodia to reduce the flow of refugees across its borders.

Sources reported today that the Thai Supreme Command has told the United Nations that 180,000 people have entered its territory in recent months from Cambodia or are gathered just across the border. Tomorrow the Thai army will begin moving 90,000 of them to a new refugee center away from the border.