Three U.S. military advisers have been relieved of their duties for being involved in a "tactical operation" in which another adviser, Staff Sgt. Jay T. Stanley of Towson, Md., was wounded Wednesday, according to a statement issued today by the U.S. Embassy here.

Two UH1H "Huey" helicopters carrying at least five U.S. advisers and Salvadoran personnel were involved in the operation, according to a statement issued by the U.S. Embassy here, and a U.S. military source said tonight that two of those dismissed were warrant officers serving here as helicopter instructor pilots. Although the mission of one of the helicopters was not disclosed, the embassy statement said the helicopter carrying Stanley was on a mission to establish direct radio contact with a Salvadoran Army unit on a "tactical operation."

The incident comes as the pace of El Salvador's civil war is stepping up dramatically and a series of apparent rebel successes has raised questions here and in Washington about the extent to which the Salvadoran Army is willing or able to fight on its own. A top Salvadoran field commander said in an interview this morning that the currently intensified fighting is becoming a test, for the Army as well as the guerrillas, of "who will wear out first."

Although embassy officials were cryptic about the exact circumstances of Stanley's wounding and today reversed some of their earlier statements on the case, what is known about the incident suggests that at least several U.S. advisers have come a great deal closer to active combat than U.S. officials previously had acknowledged.

Administrative rules limit the number of U.S. advisers in the country to 55 and prohibit them from carrying any weapon larger than a side arm or from participating in any intentional way in what is described by the embassy's statement today as "operational" missions.

The Reagan administration is asking for an increase in military aid from $26.3 million to $86.3 million in fiscal 1983 and senior administration officials also are pressing for an increase in U.S. military planners and trainers, particularly at the provincial and brigade levels.

A year ago a U.S. adviser, Lt. Col. Harry Melander, was ordered out of the country by U.S. Ambassador Deane R. Hinton because he and several of the men in his command were filmed carrying M16 automatic rifles near a temporary bridge.

The current incident is apparently more serious. On Wednesday, the two helicopters carrying the Americans went on operational missions near the strategic Cuscatlan Bridge in the area near a major guerrilla operation in Usulutan province, according to the embassy. Both helicopters drew fire, the statement said.

"According with standing instructions, American trainers should not have participated in such an operation," it added.

The two warrant officers, who were nearing the end of their normal tour of duty here, normally fly in the right-hand front seat of the helicopter when they are teaching Salvadorans, according to a well-informed U.S. military source. In that position, they have full access to the helicopter's controls but it could not be determined immediately whether either of the warrant officers were in such a position on Wednesday.

The third soldier relieved of duty was a master sergeant, who appears to have ordered Stanley on the mission. Stanley was part of a small team here commanded by a master sergeant, according to U.S. officials. U.S. officials said the three would leave the country on the "next available military flight."

The embassy statement today said that Stanley, who will be leaving for treatment of his wound, "has not been relieved because his actions were pursuant to orders from his team leader, who has been relieved."

Embassy officials declined all comment on the mission of the second helicopter, and the exact intent and activity of the one in which Stanley was flying remains unclear.

The original embassy story was that Stanley and three other U.S. advisers had gone to the bridge to repair a radio relay station. Their Salvadoran pilot was said to have spotted a traffic tie-up nearby and when he brought the helicopter down to see what was happening they were fired on. A Salvadoran door gunner was also injured, according to the embassy.

Today, the embassy said, "We have determined that the primary purpose of the flight on which Sgt. Stanley was wounded was to establish direct contact later clarified by an embassy spokesman to 'direct radio contact' with a Salvadoran Army unit on a tactical operation."

According to authoritative sources, the commander of the U.S. Military Group also had flown over the same area at an altitude as low as 20 feet Tuesday.

By Wednesday the battle for Berlin, on a mountainside a few miles to the south, was drawing to a close, and guerrilla troops were moving north from the scene. Some military sources speculate that their objectives may have been the bridge.

According to a western military observer, the column was spotted by the kind of small patrol that the U.S. advisers have encouraged the Salvadoran military to employ, air strikes were then called in, paratroopers were deployed on the ground from near the bridge and, as the observer put it, the result was "60 dead Gs," or guerrillas.

The extent to which the two sets of U.S. advisers on the helicopters aided in the success of that maneuver could not be determined.

In an interview this morning, Col. Domingo Monterrosa, commander of the U.S.-trained Atlacatl rapid reaction battalion, said that the guerrillas' recent moves have been intended mainly as propaganda ploys and do not represent significant gains in military strength as the rebels claim.

But Monterrosa said the Army has begun a process of testing the rebels to see "who wears out first" in a process of sustained combat.

Instead of pulling the U.S.-trained troops out of Morazan province, a guerrilla stronghold where they have been involved in heavy fighting since the middle of January, one of the three crack battalions will be left there, according to Salvadoran military sources, to continue patrols and attempt to control areas that the rebels have regained.