Three years ago on Christmas, two columns of Vietnamese troops spearheaded by tanks drove into neighboring Cambodia at the head of a massive invasion force. Less than two weeks later, the vanguard of this Vietnamese blitzkrieg rolled into the capital of Phnom Penh and installed a new government.

The consequences of that invasion continue to preoccupy Southeast Asia today, shaping relations among the states of the region and between each of them and the rest of the world.

The Vietnamese occupation of Cambodia has wider implications, however, for it embodies the larger conflict between China and the Soviet Union. At the same time, it illustrates the demise of the U.S. role in the region following the Vietnam War. As far as Cambodia is concerned, that role has been reduced to following the lead of the region's five-member grouping of noncommunist states, the Association of Southeast Asian Nations (ASEAN).

For ASEAN and Western countries, the problem has been complicated by the dilemma of supporting a government that most of them find abhorrent: the ousted Communist Khmer Rouge, which is blamed for the deaths of1 million to 2 million Cambodians during its 1975-79 rule.

The countries feel their diplomatic and political support is necessary because the Khmer Rouge, backed by China, fields the most effective resistance to the Vietnamese occupation. As the holder of Cambodia's U.N. seat, the Khmer Rouge also serves to deny international recognition to Hanoi's handpicked government in Phnom Penh.

Now, however, as the Vietnamese occupation drags into its fourth year, the Cambodian issue shows signs of becoming even messier.

A concerted ASEAN effort to spruce up the image of the Khmer Rouge government appears to be running into trouble. Government changes in Phnom Penh, while still mysterious, seem to indicate a tightening of hard-line Vietnamese control over the Cambodian administration. Moreover, there are signs of preparations for intensified fighting during the current dry season.

"It looks as if the Vietnamese are putting themselves into position to cope better with increased Khmer Rouge activity," said a Western diplomat who monitors Cambodia closely.

He said there was no sign yet that the Vietnamese were preparing a major offensive against the 30,000 to 40,000 Khmer Rouge guerrillas during the dry season, which lasts roughly from November to April. Rather, the diplomat said, the Vietnamese seemed to be moving toward smaller scale operations aimed at regaining the initiative in some areas.

After reaching a low point following Vietnam's invasion during the 1978-79 dry season, Khmer Rouge military capabilities began to recover toward the end of 1979. By the time of the 1980 summer monsoon, the Khmer Rouge was harassing the Vietnamese with small-scale attacks. The 1980-81 dry season saw Khmer Rouge operations in larger units.

With the Vietnamese forces suffering logistic and morale problems, diplomats said, the Khmer Rouge this year were able to take the initiative.

The Cambodian war in effect reached a stalemate, the officials said, with the Vietnamese unable to wipe out the Khmer Rouge, and the guerrillas incapable of driving out the occupiers.

Now the Khmer Rouge are vowing to raise significantly the level of fighting. Specifically, diplomats said, the guerrilla troops under ousted Cambodian dictator Pol Pot have indicated plans to start large-scale operations in western and northwestern Cambodia.

Besides tying down a substantial portion of Hanoi's forces in guarding roads, bridges and other installations, the Khmer Rouge attacks have obliged the Vietnamese to rely more on Cambodian waterways for deliveries of military supplies and to accept greater Soviet assistance for the Phnom Penh government's Army. Hanoi has been trying to build up this Army and give it a greater combat role, apparently without much success so far.

Based on reports of Vietnamese movements, diplomats expect Hanoi's Army to intensify operations in northwestern and central Cambodia in efforts to seal sections of the Thai-Cambodian border and secure Route 6. This road running north of the Tonle Sap Lake frequently has been cut by the Khmer Rouge.

Further complicating the situation is the replacement earlier this month of Pen Sovan as head of the Phnom Penh government's ruling communist party, ostensibly for health reasons. President Heng Samrin assumed the key post, surprising many diplomats who considered Pen Sovan the government's strongman.

Since then, Pen Sovan's fate has remained unknown. Western diplomats and Thai officials have been unable to confirm rumors circulating among relief agencies in Phnom Penh that Pen Sovan and about 20 other prominent Cambodians have been arrested. Various other theories abound.

Diplomats here do not rule out that Pen Sovan, 46, may indeed be seriously ill. They note unexplained absences of more than five weeks in the Soviet Union during the summer and eight days in Bulgaria in the fall. Furthermore, it is not clear why Pen Sovan has not also been formally relieved of his post as premier.

The latest theory among diplomats is that the Vietnamese got rid of their erstwhile protege because he was showing signs of independence and encouraging closer direct ties with Moscow.

Western relief officials in Phnom Penh have noticed signs of Vietnamese suspicions that the Soviets were trying to bypass Hanoi and develop an independent relationship with the Cambodian government. Although Moscow essentially bankrolls the Vietnamese war effort in Cambodia, Hanoi jealously guards its control of the Phnom Penh government.

"There's no way the Phnom Penh government could get rid of Pen Sovan without the Vietnamese approving it," one Western diplomat said. "If any regime deserves the epithet of puppet regime, that's it. The Vietnamese own the party, the government and the Army."

If this theory of Pen Sovan's demise were true, it would indicate an even more uncompromising stance by Vietnam on negotiating an end to its occupation. With their own problems in Afghanistan and Poland, economic difficulties at home and the heavy burden of aid to Vietnam, the Soviets are seen as more amenable to a compromise in Cambodia.

However, the anti-Vietnamese resistance groups show no sign of getting together in a way that would make Hanoi consider such a compromise. ASEAN's aim is to forge a coalition of the groups, thereby diluting the role of the Khmer Rouge in Cambodia's U.N.-recognized government.

But the Khmer Rouge and a prospective coalition partner, the anticommunist Khmer People's National Liberation Front, have been denouncing each other publicly in increasingly strident terms lately despite an agreement in September to refrain from such criticism. In the latest exchange, the front reacted sharply to a Khmer Rouge statement that the front's leader, former Cambodian prime minister Son Sann, spent too much time abroad while his followers along the Thai-Cambodian border ran black-market operations.

In a Dec. 23 press statement, the front said its forces were fighting in the Cambodian interior, where the Khmer Rouge had no popular support. It called the Khmer Rouge statement "false, deceitful and calumnious."

The Chinese-supported Khmer Rouge appear to have been laying the groundwork lately to reject ASEAN proposals for a loose coalition government including the Khmer Rouge, Son Sann's front and a faction led by former Cambodian head of state Prince Norodom Sihanouk. The maneuvering to form a coalition underscores the key role of China in the Cambodian conflict.

However, the Chinese seem to have a different strategy toward the Cambodian conflict than ASEAN, viewing the problem mainly as a way to "bleed" Vietnam by bogging down its forces in an open-ended war. This also ties up resources of the Soviet Union, Peking's principal rival for influence in Indochina.

While China wants to see Vietnam drained by a long war of attrition in Cambodia, the ASEAN countries seek an early end to the conflict by offering Hanoi a chance to deal with a coalition in which the Khmer Rouge would be a junior partner, minimizing Chinese influence.

Having banked heavily on such a coalition to make the ousted Khmer Rouge government internationally more presentable, ASEAN now faces a potentially serious diplomatic setback if expected Khmer Rouge counterproposals prove unacceptable.

Accordingly, ASEAN members are trying to persuade Peking to pressure the Khmer Rouge into accepting the coalition proposals, which have already been endorsed by the two noncommunist parties.

As the issue unfolds, Washington has been left largely on the sidelines.

"The United States after the Vietnam War is irrelevant," an ASEAN foreign minister said bluntly in discussing Cambodia recently. "It is a question of Sino-Soviet rivalry."