Fifteen U.S. military personnel, including the three killed Wednesday, have died in El Salvador since Washington sent uniformed advisers there a decade ago to help the government fight its civil war against leftist guerrillas.

The military training program, first proposed in March 1980 by the Carter administration, was launched despite objections by critics who argued it would lead to "another Vietnam," inevitably dragging the United States into higher levels of military support.

An informal agreement was reached in 1981 between the Reagan administration and Congress to limit the number of advisers to no more than 55. But some U.S. military and congressional officials have long described that limit as a misnomer, saying the U.S. military usually has 105 to 125 people operating in El Salvador.

Many of the trainers work in OPATS, two- or three-man Operation Planning Assistance Teams assigned to Salvadoran units. Those teams, and the remaining U.S. personnel in the country, are backed in part by another 1,200 Pentagon personnel stationed in Honduras, where the three men killed Wednesday were based.

In reaching the agreement with the executive branch, Congress exempted several categories of Defense Department representatives from the limit, including military trainers, the embassy military group and Marine guards and the defense attache. The military has also used other loopholes, including "temporary duty assignments," to exceed the formal 55-person limit. For example, the dozen Special Forces members trapped in the Sheraton Hotel during a December 1989 offensive were there on temporary duty and not officially counted.

The number of U.S. service personnel killed as a result of military confrontations has been lower than critics feared.

Although U.S. uniformed advisers have come under fire many times, especially during rebel attacks on Salvadoran bases, it was not until September 1987 that the first U.S. combat fatality was reported: Staff Sgt. Gregory Fronius, 27, of Connellsville, Pa.

The State Department yesterday listed 11 other military personnel as having died in El Salvador, including those killed in accidents.

In 1983, Lt. Cmdr. Albert A. Schaufelberger III, deputy commander of the U.S. Military Group, was shot to death as he sat in his car waiting for a friend on the grounds of Central American University in San Salvador.

Four off-duty and out-of-uniform Marines were killed in 1985 in a terrorist attack when men armed with automatic rifles sprayed a string of outdoor cafes in the capital. Thirteen people were killed and at least 15 wounded in that attack, although U.S. officials believe the rebels were looking for the Marines, who were known to frequent the cafes.

Six more U.S. military personnel died in a helicopter crash in 1987 that was described as an accident in bad weather and not the result of rebel fire. Wednesday's deaths were the first in a helicopter downed by rebel fire, sources said.