This weekend's elections in El Salvador will open a period of intensive diplomacy on several fronts to explore negotiated solutions for the deepening problems of Central America. But there is little optimism here that these efforts will succeed.

Two sets of negotiations--between the United States and Cuba and the United States and Nicaragua--are described by informed officials as nearly certain to take place in the weeks ahead. The United States is working on ideas and suggestions for a third set of talks, involving a newly elected civilian leadership in El Salvador and that country's insurgent left, but there is much less confidence that this dialogue will take place.

A flurry of diplomatic maneuvering, public and private, in recent weeks has set the stage for the talks. The most public of the maneuvers, the "Mexican initiative" announced by President Jose Lopez Portillo Feb. 21, was the subject of meetings between Secretary of State Alexander M. Haig Jr. and Foreign Minister Jorge Castaneda on March 6 and 14, and exploratory trips by Castaneda to Havana and Managua within the past week.

A confidential effort, until news of it leaked in the past several days, was a visit to Havana in early March by Gen. Vernon Walters, Haig's roving ambassador and secret emissary. While not officially confirming the Walters mission, apparently because of a U.S.-Cuban agreement not to do so, an informed U.S. official placed it in the category of periodic tests of Cuban intentions in a secret dialogue dating back to 1977.

There is no indication at present, according to this official, that a "grand negotiation" is likely to lead to a rapprochement with Havana. As in the past, Cuba is described as eager to discuss such bilateral issues as the U.S. trade embargo, the U.S. base at Guantanamo Bay and the upgrading of diplomatic relations, but reluctant to address the principal U.S. issues, such as Cuban aid for insurgency in the hemisphere and Cuban troops in Africa.

The issues on the two sides have been set forth in various ways, including an unannounced meeting in Mexico City last Nov. 23 between Haig and Cuban Vice President Carlos Rafael Rodriguez. Beyond this, a "disconnect" is reported between the U.S. and Cuban viewpoints while the two sides probe for further details of each other's positions.

The forthcoming diplomatic efforts can be seen as a series of concentric circles in Central America, with Cuba on the outer ring, Nicaragua on the second ring and El Salvador at the core.

The dialogue with Nicaragua began with a visit to Managua last August by Assistant Secretary of State Thomas O. Enders. But this effort collapsed in October amid mutual recriminations, and tensions between Washington and Managua have risen sharply. Recently, The Washington Post and other news organizations reported that the Reagan administration has authorized secret paramilitary operations against the Nicaraguan regime.

The essential U.S. condition for a rapprochement, as stated by Enders to Nicaraguan junta coordinator Daniel Ortega in writing Aug. 31, is halting "the continued use of Nicaraguan territory to support and funnel arms to insurgent movements in the area," especially El Salvador. This remains the central issue, but there is no certainty in Washington, according to a senior U.S. official, that Nicaragua sees the question as "bargainable."

Some Nicaraguan officials continue to deny officially that their territory is being used to support the Salvadoran guerrillas. In view of this, Enders reportedly told Nicaraguan leaders last year, and U.S. officials have repeated since, that only the fact of their shutdown of support is required, not a public statement. If such action were taken, the Nicaraguans reportedly were told, the United States soon would be aware of it, a reference to U.S. intelligence monitoring.

In another aspect of a potential settlement, the United States has proposed that Nicaragua and all other countries in Central America agree not to import "heavy offensive weapons" and to reduce the number of foreign military and security advisers. Managua's acquisition of Soviet Mig fighters, which has been forecast by U.S. officials for months and is still expected, would be a major setback to the chances for a negotiated arrangement as well as a powerful symbol of a Nicaraguan connection with the Soviet bloc.

Cuban President Fidel Castro, according to a report that has reinforced the concern at high levels of the U.S. government, recently described the Migs as "on the way" to Nicaragua. Officials now do not believe the warplanes will be shipped to Managua through Cuba, however, but will take another route.

Internal negotiations between government leaders in El Salvador and the that country's insurgent left are even more delicate, in the U.S. view, than the prospective Washington-Havana and Washington-Managua talks, and the internal Salvadoran discussions are being handled with even greater circumspection.

Starting with Haig, various officials of the State Department denied in a variety of ways yesterday that the administration is shifting its position on this issue.

Haig, after a meeting with the foreign ministers of El Salvador, Honduras and Costa Rica, said the United States from the outset has favored negotiations by all parties "to participate in a democratic process."

However, he added that "what we reject is a negotiated distribution of power over the heads of the people of El Salvador."

Previously, U.S. approval for negotiations involving the Salvadoran left was in the context of arrangements for Sunday's constituent assembly elections. The left refused, however, to negotiate about participating in the elections, which it strongly opposes.

The new ingredient in U.S. thinking, as described by official sources, is the set of circumstances expected to flow from the balloting. The constituent assembly, once organized by a political majority or coalition (the latter being more likely, in the current Washington assessment), is expected to be a "self-sovereign" body that can and probably will decide to tackle a host of central questions. These include the relationship between the army and civilian authority and a "control mechanism" for military forces, a constitution for the country and future legislative and presidential elections.

All this, in the administration view, is another phase of the Salvadoran political process in which the left could be encouraged to participate. Such participation clearly would be on the government's turf and subject to its terms.

U.S. officials do not deny that they are working on suggestions about such a process for possible presentation to Salvadoran leaders following Sunday's election, but the officials refuse to discuss them before the balloting.

Whatever the specific ideas, this concept of negotiations seems clearly to be much more limited than the broad negotiating process for settling a civil war which is backed by many Latin American and European governments and, reportedly, by some centrist political figures in El Salvador.