There are clear indications that three Southeast Asian nations are preparing to supply military aid to the anticommunist guerrillas in Kampuchea (Cambodia). Most experts agree that it is extremely unlikely that such outside aid will enable the noncommunist insurgents to drive Vietnam out of their country. The noncommunists are less experienced and far fewer in number than a rival anti-Vietnamese force, the communist Khmer Rouge guerrillas.
But with more and better weapons the noncommunists could certainly have a noticeable impact. They would probably grow in numbers and effectiveness. And, as one Western diplomat points out, that would put greater strain on Vietnam's 200,000-man occupation force.
Depending on the scale of military assistance from the three Southeast Asian nations - Malaysia, Singapore, and Thailand - the ripple effects could include:
* A growth in the number of noncommunist guerrillas toward matching in numbers, if not fighting skill, some 35,000 communist Khmer Rouge who already receive arms from China.
Noncommunist insurgents could gain strength in areas throughout the country where the Khmer Rouge is unpopular, according to Cornell University specialist Steven Heder in testimony to a Senate subcommittee. Vietnam, he added, would have difficulty reinforcing its army in Kampuchea to meet this threat because it must stay prepared for any Chinese attack on Vietnam from the north.
* The rise of a credible alternative to the Khmer Rouge. In the event of talks, the Vietnam-aligned government of Heng Samrin in Phnom Penh would then have someone else to negotiate with, aside from the Khmer Rouge, which is widely disliked for the atrocities it committed while in power from 1975 to 1979.
* An enlarged choice for the ''Kampuchean in the street.'' At present Kampucheans can be told by Vietnam that the Heng Samrin government is the only force that can protect them from Khmer Rouge terror. If noncommunist guerrillas grow, more Kampucheans may see an alternative preferable both to the Khmer Rouge and to Vietnam's occupation force.
At present some 9,000 fighters led by the noncommunist former prime minister Son Sann are seeking arms from the United States and noncommunist Southeast Asia. A still smaller force led by Prince Norodom Sihanouk also wants outside support. Mr. Heder's study concluded that cooperation between the better-organized Son Sann forces and the still popular Sihanouk would maximize the prospect for insurgent growth. Talks aimed at a ''united front'' between these forces and the Khmer Rouge are continuing.
US officials say no arms aid will go to the insurgents, although food and medical aid can be provided. But three of the five members of the Association of Southeast Asian Nations (ASEAN) seem to be taking a different tack and are on the brink of supplying arms, or money to buy arms.
Malaysia, once in favor of preserving Vietnam's strength as a ''buffer'' against China, has in recent months publicly moved toward a policy of supplying military aid to the insurgents.
True, Malaysian Prime Minister Dr. Mahathir Mohamad did recently declare Malaysia will not supply military aid to the noncommunist insurgents. But this line should not necessarily be taken literally, one Western analyst suggests. It may be be camouflage to downplay Malaysia's recently acquired image as a ''hawk'' on arms aid. Outspoken Foreign Minister Tan Sri Ghazali Shafie, who has often advocated aid for the insurgents, has just led a 50-member inspection team to the Thai-Kampuchean border region.
Singapore, long considered a ''hawk'' on Vietnam, also favors arms for Kampuchean insurgents.
Thailand, seen as closest to Singapore, has looked with favor toward military aid to the insurgents. But the Thais have been ''discreet'' and avoided public posturing.
Two ASEAN powers are likely to stand aside from supplying aid. The Philippines takes a largely ''interested bystander'' position. And Indonesia strongly opposes supplying military aid to either the forces of Son Sann or Sihanouk.