China has begun arming resistance fighters loyal to former Cambodian head of state Prince Norodom Sihanouk, underlining Peking's support for a broad-based coalition to drive Vietnamese forces out of Cambodia.

But the prospects for a united front seemed to dim today with the disclosure that Son Sann, who leads one of three Cambodian factions fighting the Vietnamese, refuses to join the merger announced Sunday by Sihanouk and the Marxist Khmer Rouge.

"The coalition must be tripartite," said Sihanouk at a news conference here jointly hosted by Khmer Rouge leader Khieu Samphan. "If Son Sann is not ready to join, the coalition must wait for him."

Sihanouk, who has been seeking material support from Washington and Peking for the past year, said that Chinese arms reached 3,000 of his troops in Cambodia a few days ago. The military aid included automatic rifles, machine guns, bazookas and rocket grenades, he said.

Although Peking reportedly offered last spring to equip a small force for Sihanouk, today's disclosure is the first indication that China is in fact funneling weapons to the smallest of Cambodia's anti-Vietnamese forces.

Militarily, China's supply of light arms to Sihanouk's tiny guerrilla band along the Thai-Cambodia border is not likely to alter the balance of forces against the well-equipped Vietnamese, who number 200,000.

But politically, it is seen as a significant gesture to encourage the formation of a coalition government by giving the former monarch enough fire power to attract Son Sann and the Khmer Rouge into a united front.

"This underscores Peking's commitment to a coalition including noncommunist members and seems designed to diminish suspicions that all China wants to do is reinstall a puppet Khmer Rouge regime," said a Western diplomat.

Peking provides major military aid to the Khmer Rouge, which was ousted by Vietnamese invasion forces in January 1979. The Khmer Rouge's 30,000 guerrilla fighters are regarded by Chinese strategists as the backbone of Cambodia's resistance.

Former premier Son Sann, who heads the noncommunist Khmer People's National Liberation Front, reportedly gets limited military assistance from China for the 19,000 fighters he claims to command throughout the war-scarred nation.

Peking has given its principal support to the Khmer Rouge, but it is aware of the international distaste for the Marxist organization because of its brutal rule while in power from 1975 to 1979 when an estimated 1 million to 3 million Cambodians were killed.

China, the United States and noncommunist nations of Southeast Asia have been pushing for a coalition of anti-Vietnamese groups to shore up the prestige of the Khmer Rouge, which retains U.N. recognition as the legal government of Cambodia.

Sihanouk and Khieu Samphan took a first step Sunday by forming a union in which both sides would retain their different philosophies but base the coalition on the governing structures of the Khmer Rouge.

Although the new partners invited Son Sann to join them in Peking and complete the coalition, Sihanouk said today that he has been informed Son Sann will visit Peking to discuss military aid but will not discuss the coalition.

Sihanouk said the coalition was useless without Son Sann because of the international clamor for a united front, especially by Southeast Asian nations and the United States, which said it would only consider assisting resistance forces in Cambodia if they were united.

"We are obliged that it be a tripartite group because foreign friendly countries want us to be three, not two," said Sihanouk, who has frequently criticized the Khmer Rouge for their atrocities while in power.

Khieu Samphan, speaking through Sihanouk as an interpreter, said he was reasonably optimistic about forming a three-way coalition if all three sides can get together to discuss their common interests.

"We have differences, but we have a common goal, which is to liberate our country and get the Vietnamese out of our homeland as soon as possible," Khieu Samphan said.