The leaders of three Cambodian resistance groups battling the Vietnamese occupation of their country began a series of meetings today aimed at submerging their differences in favor of a common stand against Hanoi.

Former Cambodian head of state Prince Norodom Sihanouk, and his onetime premier, Son Sann, said they would meet again Friday with the head of the communist Khmer Rouge, Khieu Samphan, to sign a joint declaration.

If the signing goes ahead, it would represent a small step toward the formation of a united front against the Vietnamese. Such a front has long been promoted by Southeast Asia's noncommunist nations as a way to increase pressure on Hanoi to withdraw its estimated 200,000 occupation troops from Cambodia. But up to now, the differences among the three groups, especially between the Khmer Rouge and Son Sann's anticommunist Khmer People's National Liberation Front, have blocked any joint stand.

Although the three men have all met each other in the past, a dinner meeting tonight at Singapore's presidential palace marked the first time that all three have come together.

Speaking to reporters late tonight after the apparently relaxed dinner, the prince said that both Khieu Samphan and Son Sann accepted the joint declaration, but that a final text would be reexamined Friday and then shown to the prime minister of Singapore, Lee Kwan Yew.

Although Singapore has been trying to play down its role in sponsoring the conference, the government here appeared to have a strong hand in getting the three sides together.