The United States, its Southeast Asian allies and the People's Republic of China, in a tacit alliance, are backing creation of a new "third force" leadership for the anti-Vietnamese resistance in Cambodia.

The most likely and prominent operational leader, former Cambodian prime minister Son Sann, has been in Washington for more than a week seeking and apparently obtaining political support of the Reagan administration. U.S. military and financial support, while not promised at this stage, has pointedly not been ruled out for the future.

China, in a development related to these maneuverings, is reported to have delivered its first major shipment of arms to the border camps of Son Sann's Khmer People's National Liberation Front in the last two weeks. The arms are reported to have crossed the territory of Thailand with little attempt at concealment.

The object of the exercise is a political alliance between Son Sann's non-communist movement and the Khmer Rouge communist movement of Pol Pot, with some chance it would be under the overall patronage of Prince Norodom Sihanouk as chief of state.

The creation on paper or in fact of such a "third force" could greatly improve international backing, and possibly internal Cambodian backing, of the anti-Vietnamese resistance.

The Carter administration, embarrassed by having to support the much-despised Pol Pot force in the United Nations as the alternative to the Vietnamese-backed Phnom Penh regime, sought inconclusively to improve the image of the Cambodian resistance.

The Reagan administration, in a decision made in its early weeks in office but never announced, decided to give a higher public profile to U.S. contacts with Son Sann to encourage him in a resistance leadership role.

A meeting between the 69-year-old Cambodian and U.S. Ambassador to Thailand Morton I. Abramowitz on March 5 was the first public step. Another step is the current "private visit" to Washington by Son Sann, who is seeing Undersecretary of State Walter Stoessel, Assistant Secretary of State-designate John H. Holdridge and other officials.

In an interview, Son Sann confirmed that he held discussions in January with Khieu Samphan, prime minister of Pol Pot's Democratic Kampuchea organization, about forging a unified resistance. Son Sann said he was offered the prime ministership of Democratic Kampuchea but that the communist emissary balked at his demand for non-communist aides in other high posts.

Making clear that a collaborative effort with Pol Pot's forces is still under consideration, Son Sann confirmed reports from diplomatic sources that he has set two basic conditions:

If he joins Pol Pot's forces, he must have the prime ministership and a majority of the other Cabinet portfolios in the new force, which probably would retain the name of Democratic Kampuchea.

Under such an arrangement, DK headquarters probably would shift from the zone dominated by Pol Pot near the Cambodian-Thai border to the zone dominated by Son Sann's non-communist force.

Before joining any coalition with Pol Pot, Son Sann's military forces must be strengthened so they will not be swallowed up by their communist allies.

In discussions with Asian and American diplomats, Son Sann has used the analogy of a man, himself, joining with the communist tiger. A man must have a strong stick before getting into the cage with a tiger, Son Sann insisted.

Son Sann did not confirm a news agency report from Bangkok that his border forces received Chinese rifles, mortars and antitank rockets other than to say it would be "good news" if accurate. Independent sources said Chinese weapons are being supplied.

Such a Chinese supply, facilitated by Thailand, would be a powerful signal from those two countries that Son Sann's conditions for joining the DK are being taken seriously.

According to a report from the Chinese capital yesterday, the official Beijing Review said in a commentary that "China supports the organization of non-communist armed forces in Kampuchea."

The commentary called for coordinated action by "the various armed forces" resisting the Vietnamese and appealed to "countries upholding justice" to grant moral and material support to "Kampuchean resistance."

State Department estimates credit Vietnam with about 200,000 troops in Cambodia, Pol Pot's forces with 30,000 to 35,000 men and Son Sann's Liberation Front with 3,000 to 5,000 fighters. The Front troops include 300-500 men in an organization loyal to Sihanouk.

Relations between Son Sann and Sihanouk are reported to be complex and difficult, as is often the case with those who deal with the mercurial former Cambodian monarch. Son Sann said neither he nor his senior aides has had recent conversations with Sihanouk or his top aides. J. Stapleton Roy, acting U.S. ambassador in Peking, met Sihanouk there about 10 days ago.

Under leadership of Thailand, the non-communist Southeast Asian nations have pushed for convening of an international conference on Cambodia under U.N. sponsorship. Thai Foreign Minister Siddhi Savetsila said here last week that U.N. Secretary General Kurt Waldheim has agreed in principle to convene such a conference, which may be held in July in Vienna.

According to Son Sann, a merger with Democratic Kampuchea is possible before a July international conference, if his conditions are swiftly met.

Aides to Son Sann said a meeting here Sunday of 300 Cambodians from throughout the United States, while condemning both the Vietnamese "puppets" in Phnom Penh and the "murderous" Pol Pot forces, gave unrestricted support to the program of Son Sann's Liberation Front.