An Italian court acquitted three Bulgarians and three Turks today of charges of plotting with Turkish gunman Mehmet Ali Agca to assassinate Pope John Paul II in May 1981.

Announcing the verdicts reached by a jury made up of two judges and six lay jurors at the end of a 10-month trial, presiding Judge Severino Santiapichi said that there was not enough evidence to convict the defendants of the conspiracy charges. Under the Italian legal system, this formula is considered a compromise verdict, a step short of a full acquittal on the grounds of proven innocence.

Today's verdicts effectively meant that, despite an extremely complex judicial investigation lasting almost five years, Italian magistrates are still baffled about the circumstances surrounding Agca's attack on the pope. They have been unable to prove allegations, made by the prosecutor who handled the pretrial investigation, of a Soviet Bloc connection to the assassination attempt or to identify any of Agca's accomplices beyond reasonable doubt.

Agca, 28, who shot and seriously wounded the pope in St. Peter's Square, had depicted the defendants as his accomplices in the assassination attempt. In pretrial testimony, he said that Turkish intermediaries had paid him the equivalent of $1.2 million to murder the Polish-born pontiff on behalf of the Bulgarian secret service.

Lawyers for Sergei Antonov, the one Bulgarian in Italian custody, said they would appeal today's verdict in the hope of eventually winning a full acquittal for their client.

Antonov, 37, the former deputy manager in Rome for the Bulgarian state airline Balkanair, has been held in prison or house arrest in Italy since Nov. 25, 1982. "I am happy that Antonov will be able finally to go free after three years of unjustified imprisonment. But I don't like this sort of compromise verdict, which is unique to Italy," said Giuseppe Consolo, the Italian defense lawyer for the Bulgarian defendants.

Bulgaria's state-run news agency BTA said the verdict showed that the so-called Bulgarian connection "existed solely in the writings of professional anticommunists . . . political provocateurs and experts in subversion and psychological warfare," The Associated Press reported from Vienna.

[The Soviet news agency Tass said: "The West's reactionary quarters failed to take advantage of the case of the attempted pope murder to further their sordid aims of smearing the Bulgarian nationals and thereby besmirching socialist countries," United Press International reported from Moscow.]

Italian magistrates blocked Bulgarian plans to fly Antonov back to Sofia this afternoon on a regular Balkanair flight on the grounds that the necessary paperwork had not been completed.

Prosecutor Antonio Marini said that he had no plans to appeal the court's verdicts on the Bulgarian defendants, since they were in line with his final recommendations to the jury last month. He said, however, that he probably would appeal the acquittals of the Turkish defendants, since he had earlier called for convictions.

Marini linked the acquittals to the bizarre behavior of Agca, who reversed himself repeatedly during the trial and made grandiose claims -- including that he was Jesus Christ -- that undercut the credibility of his testimony. Marini said he would have supported psychiatric tests of his star witness to determine whether Agca is insane or has simply been pretending to be crazy to disrupt the trial.

It was largely on the basis of Agca's statements during a two-year pretrial investigation that the state brought charges of conspiracy to murder against the Bulgarian and Turkish defendants.

The other Bulgarian defendants acquitted today were Todor Aivazov, 42, and Lt. Col. Zhelio Vasilev, 43, the cashier and assistant military attache at the Bulgarian Embassy in Rome at the time of the assassination attempt. Both men returned to Bulgaria before they could be arrested and refused to attend the present trial on the grounds of diplomatic immunity.

Also acquitted was Musa Serdar Celebi, 34, the head of a right-wing Turkish emigre organization in West Germany accused by Agca of providing logistical support for the assassination attempt. The court withdrew a warrant for the arrest of another right-wing Turk, Oral Celik, 26.

Omer Bagci, 40, a junior member of Celebi's organization, was found guilty of smuggling the gun used by Agca to shoot the pope into Italy and was sentenced to three years' imprisonment. But the court cleared him of the principal charge of conspiracy on the grounds that there was no proof he had any knowledge of Agca's intentions. Judicial sources said Bagci will not have to serve the term because he already has been jailed for a longer period since being extradited from Switzerland on Oct. 15, 1982.

A fourth Turkish defendant, Bekir Celenk, died on Oct. 14, 1985, of a heart attack in a Turkish prison.

Agca, who already is serving a life sentence for the attempted murder of the pope, was sentenced to a further one-year prison term on the charge of smuggling a gun into Italy.