The three military-backed governments of El Salvador, Guatemala and Honduras are moving to coordinate military strategy against growing leftist guerrilla activities in the region, according to Latin American diplomatic sources here.

Although no formal accord has been signed, the sources said, the military commands of the three countries have agreed to step up exchange of intelligence and are contemplating a defense alliance under which Guatemala and Honduras would help the Salvadoran forces confront future guerrilla offensives.

Guatemala and Honduras already have stationed heavy troop concentrations along their borders with El Salvador, but have denied frequent reports that their troops have conducted brief operations across the frontier. Other contingencies under discussion include regional actions in case Salvadoran or Guatemalan guerrillas flee into neighboring territory, the sources said.

A flurry of high-level talks among the three countries climaxed last week when the Guatemalan president, Gen. Romeo Lucas Garcia, and Honduran Army Chief of Staff Mario Chincilla visited the Salvadoran capital on the same day. The brief communique released before Lucas' departure did not mention a joint military strategy.

A few days before the meeting, however, the army staff chiefs of El Salvador and Guatemala publicly called for military "coordination" among the three countries while Guatemala's Gen. Benedicto Lucas, the president's brother, urged a formal fusion of the three armed forces "to prevent a communist takeover of Central America."

El Salvador, which has been fighting a civil war against leftist guerrillas for nearly a year, has the largest military force of the three countries, with 20,000 troops. Guatemala's military forces currently number about 15,000, and Honduras has 12,000.

All three governments receive varying degrees of U.S. support and military aid. American diplomats in the region, contacted by telephone, said no U.S. representatives were present at the talks Oct. 12 among the three military commands.

Latin diplomatic sources here, however, point out that the United States has been closely involved in the recent past in efforts at coordination and is known to perceive the region as of crucial security interest.

U.S. pressure was a key element in the successful negotiation of a peace treaty signed last December between El Salvador and Honduras, ending a decade in which the two technically were at war and clearing the way for coordination of military efforts against guerrillas along their border.

At the same time the United States has sent military trainers and doubled to $10 million military assistance to Honduras -- the most stable internally of the three countries. This month, U.S. forces held joint maneuvers with the Honduran military to practice sealing off the country by sea -- to stop possible Cuban boats carrying arms destined for guerrillas in El Salvador -- and by land.

In many ways, cooperation among the three countries is more a marriage of convenience and perception of a common threat than a genuine desire to pool their resources.

"There is no great trust and affection among the three," one U.S. diplomat said. "The Hondurans think the Guatemalans are wild, and El Salvador and Honduras are long-time enemies. But the three now have a number of common interests. They are all worried about leftist guerrillas and arms traffic, and all three are concerned about the military buildup in Nicaragua."

The military rulers in all three countries see Nicaragua's Sandinistas and their close ties with Cuba as a growing threat to their national security. On the other hand, Honduras is home to thousands of former Nicaraguan National Guard soldiers who fled before the Sandinista takeover. The exiles, who have support from sectors of the Honduran military, often launch small attacks on Nicaraguan farms and Army patrols. They have announced plans for a full-scale invasion, which Nicaragua cites as the reason for its military buildup and its training of a large militia.

In recent weeks the three countries to the north of Nicaragua have been stepping up their verbal attacks on the Sandinistas, who they claim still provide arms and other support to the Salvadoran guerrillas. In all three countries there is talk of a break in diplomatic relations with Managua.

In Guatemala, whose military is considered most right-wing, at least 30 Nicaraguans living there or just visiting have disappeared during the past two years.