One of the ruling Sandinistas' staunchest democratic allies in Latin America, Venezuela, cautioned them at their third anniversary celebration today that they must "confront their conscience" about whether to honor old promises to establish a pluralistic democracy here.

The Nicaraguan leaders replied that their main confrontation at the moment is with a U.S. government attempting to undermine their government economically, politically and by supporting military action against them.

Neither Venezuelan President Luis Herrera Campins nor Nicaraguan junta member Daniel Ortega had anything good to say about the Reagan administration or its current policies toward Latin America. What unity they found was in their attacks on U.S. policy toward the Falklands war.

The desultory anniversary celebration in a dusty, rural town about 15 miles west of Managua where some of the heaviest fighting took place during the war to overthrow dictator Anastasio Somoza seemed to underscore the changes of the past three years. The revolution, once marked by a contagious enthusiasm that united this country's people and inspired sympathy throughout the world, has begun to appear drab and embattled, its ranks divided.

In today's speeches, however, much responsibility for the country's troubles was laid at the door of the Reagan administration because of its policies not just toward Nicaragua but toward Latin America as a whole.

The Venezuelan president said the Falklands war has caused Latin Americans to reflect on and to review their relations with the United States.

Herrera Campins, whose government often has backed Washington on controversial regional questions such as the civil war in El Salvador, attacked the Reagan administration for first presenting itself as a mediator, then backing "colonialist" Britain against Argentina in the South Atlantic conflict.

Ortega's speech was an even sharper indictment of the United States, and halfway through it U.S. Ambassador Anthony Quainton walked off the grandstand.

Ortega accused both the U.S. Central Intelligence Agency and the Honduran Army of backing a "bloody, silent invasion" of Nicaragua by former members of Somoza's National Guard.

Ortega estimated that more than 2,000 anti-Sandinista rebels are now operating along the border and inside the country. He said that since July 4 they have staged at least 18 attacks and killed more than 50 government soldiers.

As recently as 5:20 this morning, Ortega said, a twin-engine airplane flew over the Pacific Coast port of Corinto and attempted unsuccessfully to blow up a fuel storage area with two rockets.

"These are not imaginings," said Ortega. Honduras has denied aiding the insurgents.

Ortega cited news reports last spring of a $19 million covert destabilization plan against the Sandinistas approved by President Reagan, the positioning of an intelligence-gathering U.S. destroyer off Nicaragua's coast and the formation of a "misnamed Central American Democratic Community" including the Guatemalan military regime but excluding Nicaragua as examples of more subtle aggression.

Herrera Campins, whose predecessor as president gave the Sandinistas political backing and much-needed arms during the insurrection, repeatedly referred to the pledges made then by revolutionary leaders that a pluralistic democracy with free elections would be established here.

On the first anniversary of their triumph, the Sandinistas dismissed the idea of "conventional" elections and announced a schedule to prepare for a nationwide vote by 1985.

"On this day," said Herrera Campins, "Nicaragua is confronting its conscience."

He also cautioned the Sandinista leaders, who have emphasized their warm relations with communist countries and revolutionary movements, that they should avoid relying on either superpower and look instead to their Latin American allies to help them live "in peace and liberty."