Nicaragua yesterday expelled three U.S. diplomats, accusing them of plotting to kill officials of the Sandinista government, including Foreign Minister Miguel D'Escoto.

The U.S. Embassy immediately protested the expulsions and dismissed the charges as "absurd." Asked if the United States was planning to retaliate by expelling Nicaraguan diplomats here, a State Department spokesman said, "We have under consideration what steps might be appropriate in the light of this action."

White House deputy press secretary Larry Speakes commented on the Nicaraguan charges, saying the administration "totally rejects their conclusions and their charges in that matter."

Nicaraguan officials held two news conferences in Managua yesterday to present detailed accounts of an alleged Central Intelligence Agency plot to assassinate Sandinista leaders and otherwise "destabilize" the government. They presented videotapes of the U.S. diplomats meeting with Nicaraguans and exhibits of alleged espionage paraphernalia.

The Nicaraguan charges and the expulsions brought relations between Washington and the revolutionary Sandinista government to their lowest point since the Sandinistas overthrew dictator Anastasio Somoza almost four years ago.

The United States has criticized the Sandinistas for supporting guerrillas in El Salvador and restricting political freedoms at home. The Nicaraguans, in turn, have accused the United States of supporting armed groups seeking to overthrow the revolutionary government.

Reports from Managua by news services identified the three diplomats declared persona non grata as Linda Pfeifel, a political affairs officer; David Noble Greig, first secretary, and Ermila Loreta Rodriguez, second secretary. The order, issued at 10 p.m. Sunday, gave them 24 hours to leave the country.

Saul Arana, director of the North American Department of the Nicaraguan Foreign Ministry, said the accusations against the three diplomats were first detailed at a press conference by Lenin Cerna, head of the Interior Ministry's Department of State Security.

Arana, who ran the Nicaraguan Embassy in Washington immediately after the Sandinista takeover, said two Nicaraguans were also implicated in the alleged plots. One of them, attorney Carlos Icaza, took refuge in the Venezuelan Embassy, he said. The other, Mario Castillo, head of the youth organization of the Democratic Conservative Party, was arrested, he added.

Susan Morgan, a British free-lance journalist who attended the news conference, gave the following account. The star witness was Marlene Moncada, 28, a Nicaraguan consular official who said she had been recruited by the CIA last year while working in Honduras. Moncada said she reported the effort to recruit her to Nicaraguan authorities and agreed to work as a double agent.

When she returned to Managua, Moncada said, she was trained by Pfeifel in spying techniques including the sending of coded messages, radio transmissions and dropping information in secure sites. She said Pfeifel asked her to study Foreign Minister D'Escoto's daily routine and eventually gave her a poisoned bottle of a liqueur to give to him.

Cerna, the director of state security, took up the theme of CIA involvement by the three diplomats, saying that Rodriquez was directly responsible for the assassination plot against D'Escoto, that Greig was the CIA station chief in Nicaragua and that Pfeifel was a CIA official with special responsibility for contacting opposition political parties and right-wing trade unions for counterrevolutionary activities.

His briefing was punctuated with the showing of a short videotape with a dramatic James-Bond-style sound track proporting to show the meeting of Pfeifel with contacts and other alleged CIA agents in a local Managua restaurant. Blown-up black-and-white photographs covered the walls showing the picking-up of the poisoned bottle from the base of a tree trunk by Moncada, photographs of meetings between embassy officials and their contacts and, on another wall, photographs of spying equipment including pads of codes, two carved wooden statuettes--one hollow--in which messages were allegedly transmitted, a radio used for transmitting coded messages and paper that could be swallowed.

Morgan reported that the elaborate preparations for the press conference with Moncada, and a later one with another double agent, Army Lt. Jorge Rostran, caused speculation among diplomats and other observers about the Sandinistas' motives in making the accusations at this time.

"The Nicaraguans never do anything on the spur of the moment," one western diplomat said. "All this was clearly carefully thought out."

Diplomats and opposition party sources suggested that the expulsions were connected to a crackdown on the opposition that has already begun. In addition to Castillo, Enrique Borgen, another Conservative Party leader, was arrested this morning.

Cerna said the entire network of "collaborators with the CIA" had not yet been arrested.

Nicaragua's Sandinista leaders have expressed fears that the United States would try to overthrow them since shortly after they took power in July 1979. Their fears were heightened after the election of Reagan who had criticized the Nicaraguan revolution in his campaign.

The Reagan administration first cut off aid to Nicaragua because of alleged Nicaraguan aid to the Salvadoran guerrillas, then authorized covert assistance to the Nicaraguan exile groups fighting the Sandinistas ostensibly to cut the supply lines to the Salvadorans.