United Nations, N. Y. — For the third year in a row the UN General Assembly is expected to express its opposition this week to Vietnam's presence in Cambodia. But this will not help unfreeze the deadlock remaining between the ASEAN countries (Malaysia, Singapore, Thailand, Indonesia, and the Philippines), on one side, and the Indochinese countries (Vietnam, Cambodia, and Laos), on the other over Cambodia.
An ASEAN resolution in favor of the withdrawal of foreign troops from Cambodia and of the Cambodians' right to self-determination is expected to win ''yes'' votes from 100 of the General Assembly's 156 members. Last year it drew 97 and two years ago, 91.
This vote is interpreted here as expressing the growing hostility felt by the vast majority of the United Nations against foreign interventions and occupations.
Vietnam continues to claim that the dialogue between both groups of nations should be taken out of the UN and pursued in a strictly regional context. All the problems, including the one of Cambodia, regarding the peace and stability of the area would be considered in such direct talks. Presumably, this would keep the big powers from meddling in matters regarding Southeast Asia alone.
But ASEAN countries cannot accept this approach. As one ASEAN ambassador puts it, ''It means that the Cambodian question would be put on the back burner. It means that the Heng Samrin government would be legitimized and that the talks would deal mainly with the Sino-Vietnamese problem.
''We say to Vietnam: Show your peaceful intentions first by allowing free elections to take place in Cambodia and we shall then seek ways for our nations to coexist peacefully.''
Vietnam does not, according to one of its diplomats, ''attach too much importance to the UN debate on Cambodia.'' It believes time is working for Heng Samrin in Phnom Penh and that ''sooner or later realism will prevail. In supporting the Khmer Rouge, the UN is supporting a fiction,'' the diplomat says.
While the ASEAN countries are in a tactical alliance with China regarding Cambodia, they do not share China's long-term strategy, according to one analyst here.
The ASEAN countries, for one thing, do not favor the return to power of the discredited Khmer Rouge and they favor a genuinely neutral Cambodia. China is believed to fully support the Khmer Rouge and to be seeking basically to destabilize the present Vietnamese regime, which refuses to toe the Peking line.
As long as there is no solution to the Sino-Vietnam problem, an internationally acceptable solution to the Cambodian problem is unlikely to emerge; this view is shared by many ''old Indochina hands'' here.