Three enlisted men of the Imperial Guard, the crack unit entrusted with protecting shah, killed 12 officers in the first known case of serious breakdown of military discipline in the long Iranian crisie, it was learned today.

The shootings, coming after rumors of desertions and tension within the armed forces, were seen as signs of a potential new threat to Shah Mohammad Reza Pahlavi's rule, since the army represents the last firm bastion day in mess hall at a major base only a few miles from the shah's Niavaran Palace in northern Tehran, informed sources reported.

The revelation coincided with a call for the shah's abdication by one of his moderate supporters and a devoted monarchist, former prime minister Ali Amini. He announced that he now favros a council to preserve the form of the monarchy, but ensurethat shah and his family leave Iran for the time being. This seemed to sink hopes of stitching together a coalition government, under the shah's continued rule, to prepare new elections.

The main leader of religious opposition to the shah-exiled Ayatollah Ruhollah Khomeini-has in any case consistently opposed any compromise that would leave the shah on the Peacock Throne that is the symbol of his 37 years of absolutis rule.

Informed sources who reported the shootings, in the face of official sinlence, said they injured 50 persons at the sprawling Lavizan Base, which also houses headquarters for Iranian ground forces and the American school here.

Other sources described the dead as young officers. Th fate of the assailants was not immmediately known, but observers doubted they could escape from the well-guarded compound and there were reports that all three were shot by loyal troops.

The attrack came on the second day of this week's mass demonstrations in Tehran, in which more than a million Iranians marched in what opposition pliticians called a "spontaneous referendum" demanding removal of the shah.

The prime minister, Gen. Gholam Reza Azhari, pulled troops out of most of Tehran for the two-day period to avoid bloodshed and the risk of putting troops to the test of having to shoot fellow Iranians in case of trouble. From his exile near Paris, Khomeini has for months called on the armed forces to desert the sovereign and overthrow Pahlavi rule.

Informed sources close to the military reported growing disenchantment even in the ranks of professional officers, long considered the government's pampered elite living apart from the rest of Iranian society.

Significantly, the disenchantment also has effected the air force, considered the service the most militantly in favor of the shah, according to reports reaching here from Tabriz in northwestern Iran. An unspecified number of Northrop F5As were sabotaged about a week ago at Tabriz Air Base, whic also handles F4 Phantoms. Informed sources said no one was injured in the incident.

As aave those in othe walks of Rianian life, professional army officers, in the past month especially have begun to lose their once-total respect for the shah. But informed sources insist that the breaking point-when the army or parts of it might consider keeping the shah a greater liability to national unity than forcing him out-has not yet come.

In the officer corps, the disenchantment was reported to be more serious among colonels and brigadier generals, men in their 40s, than among top commanders, who almost without exception are unquestioning loyalists in their 60s.

The Imperial Guard, made up of one armored and three infantry brigades, is thought to have its own helicopter capability and boasts between 10,000 and 12,000 men. Its 6-foot-6 commander, Lt. Gen. Baderi, reports directly to the shah.

The Guard is diviied into two parts. The Javidan Brigade, which is known as "The Immortals," is entirely professional from private up through the ranks. The Lashgari Units, apparently involved in the Monday shooting, have drafted enlisted men, but the non-commissioned officers and officers are all professional

Demonstrations in favor of the shah, encouraged by the armed forces despite their technical violation of martial law, continued meanwhile in Isfahan, Khorramashahr and other towns and cities across the country.

On the political front, National Front leader Karim Sanjabi announced that he had conferred with the shah last night for one hour. Judging from the negative tone of a communique the National Front published, Sanjabi had not changed his repeated refusal to join a coalition civillian government at this point.

Amini's call for the shah to leave his throne seemed to mark a departure from his previos efforts to act as a mediator between the shah and the opposition in hopes of forming some coalition government that would allow the monarch to remain under a constitutional arrangement that would circumscribe his sbsolute authority. At best for the shan, it held out prospects of eventually letting a shah, presumably the 18-year-old Crown Prince Reza, come back as a figure-head monarch at an unspecified time.

Observers expressed doubt whether the shah would accept Amini's proposals, at least immediately. Kambiz Yazanpanah, director general of the shah's press section, dismissed the announcement that Amini had made such proposals to the shah as "absolute fabrication."

"It is not possible," Yazanpanah said. CAPTION: Picture, GEORGE W. BALL . . . consultant on Persian Gulf issues