Former Cambodian head of state Prince Norodom Sihanouk and two other Cambodian leaders opposing the Vietnamese occupation of their country today papered over their differences and agreed to work toward the formation of a coalition government.

After two days of talks, Sihanouk, former premier Son Sann and Khmer Rouge leader Khieu Samphan signed a joint statement outlining modest areas of agreement on resisting Vietnam's 2 1/2-year-old occupation of Cambodia.

Specifically, they agreed to set up a committee to work out the basis of a coalition.

Although limited, the accord marked the first time that the three sides have been able to agree on anything since Hanoi's troops invaded Cambodia in December 1978 and installed a protege government the following month.

The joint statement thus appeared to meet the aims of countries opposing the Vietnamese occupation to show at least some progress in uniting the resistance before the U.N. General Assembly convenes this month.

At stake then will be Cambodia's U.N. seat, which is currently held by the ousted Khmer Rouge government but which Hanoi and its Soviet Bloc allies want for the Phnom Penh administration set up by the Vietnamese.

Vietnam's noncommunist neighbors in the Association of Southeast Asian Nations (ASEAN), as well as China, the United States and its Western allies, want the seat to remain in the hands of the communist Khmer Rouge government, known as Democratic Kampuchea. But they would prefer leadership changes in that government to spruce up an international image sullied by nearly four years of death and destruction in Cambodia during the Khmer Rouge's brutal rule.

The convoluted wording of the joint statement indicated that a long way remains before any actual formation of a coalition government.

The statement said Sihanouk, Son Sann and Khieu Samphan "agreed to express the desire to form a coalition government of Democratic Kampuchea with a view to continuing the struggle in all forms for the liberation of Cambodia from the Vietnamese aggressors."

The statement said the three sides further agreed to "set up an ad hoc committee to study the principles and forms enabling the achievement of the above coalition government and objectives."

It concluded that the three Cambodian leaders had agreed "that all anti-Vietnamese forces avoid any clashes among themselves . . . during the whole period of the agreement."

During a joint news conference after the signing, Sihanouk made clear that the accord has little, if any, military significance for the battle against the estimated 200,000 Vietnamese troops occupying Cambodia. He said the three resistance groups would continue to maintain separate military organizations, although they might form "a joint general staff."

The Khmer Rouge, with 30,000 to 40,000 hardened troops and a steady supply of aid from China, currently are doing most of the fighting against the Vietnamese.

Son Sann's anticommunist Khmer People's National Liberation Front, with only about 5,000 to 6,000 fighters, has been more effective politically inside Cambodia.

To avoid domination by the Khmer Rouge in a coalition, Son Sann's group has been seeking foreign aid to bring its forces up to par with the communist resistance.

While Sihanouk's Moulinaka organization has only a few hundred combatants, his international standing is considered an important addition to any possible coalition.

In answer to a question at the news conference, Sihanouk said he expects to get more military support for the resistance from the United States.

Flanked by Son Sann on his right and Khieu Samphan on his left, Sihanouk said the three parties' next step would be to appoint representatives to serve on the ad hoc committee.

He said he supported Son Sann to be prime minister of a coalition government, a nomination that the Khmer Rouge had already proposed.

Son Sann, sitting stoically throughout the press conference, said grimly, "Today is a very happy day because we agreed on the liberation of Cambodia . . . . It is a very good step, a very big step we have taken today."