Three years ago this month, every senior-level civilian official resigned from the Salvadoran government, in large part because Defense Minister Jose Guillermo Garcia would not.

Just over two years ago, after months of bloody paramilitary death-squad activity and the death of four American churchwomen, the Christian Democrats in the government were saying that there was no point going on if control of the Army remained in the hands of Garcia and his allies.

Yet the advent of the Reagan administration in Washington and what appeared at the time a major victory over leftist guerrillas in January 1981--combined with Garcia's deft political maneuvering--not only rehabilitated his reputation but made him, in effect, the standard-bearer for U.S. policy in El Salvador during the past year.

Now the general in whom the United States has invested its confidence and military aid looks severely threatened by a mutiny involving some of his most effective officers. One diplomat suggested that Garcia gained much of his current support "by default."

Garcia, by contrasting himself with right-wing ex-major Roberto D'Aubuisson, has seemed to emerge as the moderate sought by the U.S. officials here who consistently pursue the goal of creating a "centrist" government between the extremes offered by left-wing guerrillas and a radical rightist economic elite.

State Department officials in Washington have suggested in recent months that whatever Garcia's past, he learned the needs and prejudices of American policy, found they conformed with his own needs and ambitions, and then became in effect the key to its implementation here.

During the past year, Garcia has been credited by the U.S. Embassy with spearheading such American-backed initiatives as the March elections, institution of a "pluralist" government in April despite the ultra-right's victory at the polls, and salvaging of the troubled agrarian reform program in June.

Although Garcia was chief of the armed forces when a number of sensational murders allegedly were committed by his men, he has generally sidestepped even indirect responsibility for the crimes while taking credit for such investigations as have been carried out.

The inherent contradictions between Garcia's power and prestige, however, have left U.S. policy on uncertain foundations now that he appears in trouble.

Garcia's most fundamental asset was his seeming ability to keep the 30,000-man armed forces united in the cause of fighting the insurgents. But the six-day-old mutiny by the 1,200-man garrison in Cabanas province has shown deep divisions even among some of his top, handpicked commanders.

Two of them, the heads of the Air Force and the capital's powerful 1st Brigade, have sided with the rebellion. The situation remained largely stalemated there today, although the armed forces cut communications to the headquarters of mutinous Lt. Col. Sigifredo Ochoa in Sensuntepeque, 45 miles northwest of the capital.

As much of the high command maintained a low profile amid backroom efforts to reach a compromise, Garcia was faced with an institution embittered by three years of what Ochoa calls Garcia's "Machiavellian" policies.

To a considerable extent, Garcia may have been the only Salvadoran officer for Washington to turn to because he arranged it that way. In 1979, young officers attempted to short-circuit the cycle of repression and radical revolution that had brought the leftist Sandinistas to power in nearby Nicaragua. The captains and majors overthrew the repressive government of general Carlos Humberto Romero and announced a program of sweeping economic, social and political reforms.

Then-colonel Garcia, a latecomer to the movement, was regarded as somewhat conservative by many of the young officers, but they also felt someone of superior rank was needed to head the Defense Ministry.

Despite leftist accusations that Garcia was corrupt and involved with the past government's repressive policies, even the nominal leader of the youth movement and member of the "revolutionary" junta, Col. Adolfo Arnoldo Majano, said subsequently that Garcia had seemed "much more evolved" than other officers of his generation.

Majano is now in exile, forced out of the country by the Army high command.

Once in power, Garcia filled key slots with friends and schoolmates, including Eugenio Vides Casanova as head of the National Guard and Col. Francisco Moran as chief of the Treasury police. They, like Garcia, have remained in place ever since.

During the course of 1980, the defense minister moved to crush the same "military youth" that had opened his way to power. In September 1980, he issued a general order removing virtually all these "Majanistas" from significant commands.

That December, the more moderate political leaders on the left were kidnaped and murdered during one week, the American church women were killed the next, U.S. aid was suspended, and Majano was forced to resign from the junta. The Christian Democrats participating in the government then demanded the removal of several key right-wing officers, including Moran, and, according to some, Garcia, as the price for their continued presence.

But at the start of 1981, the guerrillas began an all-out military offensive to seize power. When that first major effort failed, Garcia emerged with his reputation recuscitated. His friend Moran stayed in power and for the time being the Christian Democrats stayed as well.

American policy was shifting gears as the Reagan administration came into office, with increasing emphasis on U.S. military and economic aid to stop Marxist advances here and in the rest of Central America. At the same time, Garcia was increasingly credited with implementing policy goals advocated by Washington.

Garcia has taken credit for pushing ahead and guaranteeing the progress of electoral politics in El Salvador at the same time that he claimed to be making the Army a "neutral" political force interested only in combating the guerrillas.

Although Garcia has been widely rumored to have presidential aspirations, he contrasted himself with the radical right-wing image of D'Aubuisson, now Constituent Assembly president.

The mutiny now under way is supported by officers personally close to D'Aubuisson but also by former "Majanistas."

In the past, U.S. officials have said figuratively that "Garcia speaks our language." But with the defense minister increasingly discredited by the endurance of the mutiny, there is a growing chance that the entire high command will be reshuffled in the near future.

The embassy, which actively and overtly supported the defense minister in a series of political showdowns with D'Aubuisson beginning in April, now maintains a studied neutrality toward what is described as a "strictly internal" conflict of the Salvadoran armed forces.

While Garcia is not expected to step down immediately, the groundwork has been laid for him to leave his command as soon as he can do so with some facesaving for himself and the military hierarchy.

In February, Garcia ends his 30th year of service, a time when Salvadoran officers are expected to retire even under normal circumstances. The question already being asked is who might replace him.