The revolutionary Sandinista government greeted the arrival here today of 20 Nicaraguan diplomats expelled from the United States with a mixture of indignation and fierce rhetoric.

But western diplomats said they do not expect the three-day-old diplomatic confrontation between Managua and Washington to escalate further and some noted that Nicaragua appeared unprepared for the Reagan administration's stern reaction to the expulsion Monday of three U.S. diplomats on charges of espionage, including an alleged plot to murder Foreign Minister Miguel D'Escoto.

The first to emerge from the plane was the consul from San Francisco, Aura Beteta, who raised her fist in a revolutionary salute. Her two teen-aged sons, who like many of the dependents hold dual citizenship, remained in California. There were six dependents on the commercial flight.

According to other passengers, the plane was seen off in Miami by a hostile crowd of Nicaraguan exiles shouting, "Take rice, beans and chicken back to the Nicaraguan people."

Arriving here, they were greeted by Sandinista partisans vowing to resist "imperialists" and counterrevolutionaries. D'Escoto greeted the returning consuls but had no prepared statement. Asked if he thought there might be further response to this by Nicaragua, he said, "We are studying the situation."

"We denounce the murders and then they come to say to us, 'Lower your tone, your rhetoric,' " D'Escoto told local reporters yesterday, "in that manner warning us not even to say that they're killing us. I don't understand what the sweet words could be that one could use to describe the crimes committed against our people."

But both D'Escoto and the U.S. Embassy said that plans continue for the Reagan administration's special envoy to Central America, former Democratic senator Richard Stone, to visit here Friday.

D'Escoto said that he believes Stone's conservative ideas about Nicaragua will not change and cited what he considered Stone's "crude, disrespectful attitude" the one time they had met previously at a Senate luncheon in late 1979.

But D'Escoto said that Stone should come to Nicaragua, nevertheless, so that he can visit the north of the country where U.S.-backed rebels are waging war against Sandinista forces and learn "the consequences of a criminal policy that is not limited to frustrated attempts on people's lives, but also means the day-to-day deaths of Nicaraguans."

U.S. Ambassador Anthony Quainton called the allegations by Sandinista State Security that members of his staff plotted the slow, untraceable death of D'Escoto with a bottle of poisoned Benedictine liqueur "clearly preposterous."

But with regard to the intelligence function of the three U.S. diplomats expelled Monday, Quainton limited himself to saying, "The United States government never comments on the intelligence role of individual personnel."

"We maintain contacts with a wide spectrum of Nicaraguans in all walks of life at all levels, inside the government and out, and they were doing their job and maintaining contact," Quainton said, "in an effort to understand Nicaraguan reality."

Some Western diplomats speculated that the purpose of expelling the three Americans in the first place may have been to isolate further the U.S. Embassy from contact with Nicaraguan opposition figures and to intimidate the opposition itself.

Several members of the Democratic Conservative Party, one of the last active political forces openly opposing the Sandinista government here, reportedly have been arrested or gone into hiding since the crisis began Sunday.

Carlos Icaza, a lawyer and sociologist described by one Conservative as "a kind of ex-officio counselor" to the party and accused by the Sandinistas of being the main Nicaraguan agent recruiting for the U.S. Central Intelligence Agency, reportedly has taken refuge in the Venezuelan Embassy.

Enrique Sotelo Borge, another lawyer and Conservative who acted as liaison between his party and various foreign embassies, has been arrested, according to members of the Conservative Party, along with a former member of the Conservative youth organization who reportedly has confessed to attempts at organizing terrorist cells in return for alleged CIA payments.

The U.S. Embassy first learned of the charges against its diplomats about 10:30 p.m. Sunday when a note from the Foreign Ministry was received during a reception at the ambassador's residence.

The note ordered political counselor and first secretary Linda Pfeifel, David Greig, described by an embassy spokesman as "the second most senior political officer," and Ermila Rodriguez, "another political officer," out of the country within 24 hours.

At an elaborately orchestrated press conference Monday by State Security chief Lenin Cerna, an alleged CIA-Sandinista double agent named Marlene Moncada testified about being recruited when she worked in the Nicaraguan consulate in Honduras.

Cerna exhibited such paraphernalia as code books, wooden-idol bookends with secret compartments and a notebook with paper that could be swallowed, as well as the bottle of poisoned Benedictine, allegedly given to Moncada through a secret "drop" to be passed on to D'Escoto.

The poison used eventually would have caused D'Escoto to lose his hair and eyebrows, possibly become sterile and die, Cerna said.

The evidence presented against Pfeifel appears to have little independent corroboration. She was known in the embassy and to several correspondents as a moderate who gave a balanced picture of flaws and virtues in the developing Sandinista revolution.

Greig had been named before in the Sandinista press as the alleged CIA station chief here. Rodriguez was named by Moncada as her main contact.

She appeared in Sandinista photographs and videotapes shot from three separate angles meeting with Moncada and giving her a ride at a local private school. The videotape at the televised press conference was accompanied by music suitable for a Hitchcock thriller.

Among today's arrivals was Leonor Arguello de Huper, 61, until today the consul in New York. Asked about the impact of the expulsions, she said, "Without consulates there can be no commercial relations."

She also alleged U.S. violations of human rights and said the American press was manipulated.

A diplomat here said impact of the consuls' ouster on trade probably would be minimal because regulations requiring consular involvement were made by the government here, not in Washington.