In a day of major new revelations in the papal conspiracy trial here, Turkish gunman Mehmet Ali Agca testified today for the first time that a third Turkish conspirator was with him in St. Peter's Square in Rome in May 1981 when he shot Pope John Paul II.

Agca also stated that a mysterious underworld figure with links to the Italian secret service had offered him his freedom if he cooperated with the Italian authorities. He named the alleged go-between as Francesco Pazienza, who was arrested in New York on March 4 and is wanted in Italy for crimes ranging from fraud to corruption of the secret services.

Agca's latest revelations drew gasps of astonishment from a courtroom that has almost become accustomed to sensational developments and daily changes in testimony by the prosecution's star witness.

Since the trial opened almost four weeks ago, the pope's would-be assassin has managed to implicate both the Soviet Union and a right-wing Italian Masonic lodge in the affair while asserting repeatedly that he is Jesus Christ.

In addition to Agca, the defendants now on trial are three former Bulgarian officials in Rome and four Turks who are accused of acting as his accomplices. An Italian state prosecutor has linked the assassination attempt to the Kremlin's need to suppress political and social upheavals in the pope's native Poland in the years 1980-81.

Agca's acknowledgment that three Turks were in St. Peter's Square on the day of the shooting was not volunteered but came when he was under strong pressure from the judge to explain photographic and other evidence that appeared to contradict his earlier assertions. The papal assailant, who has said that he was accompanied to the square by Bulgarian secret agents, had revealed in December 1982 that a Turkish right-wing terrorist, Oral Celik, was also present.

As late as yesterday, Agca insisted that Celik had been the only other Turk present despite photographs showing himself and two other dark-haired men glancing at each other at the moment of the shooting.

However, unlike Agca's statement about the third Turk -- which was almost dragged out of him -- the pope's assailant volunteered the information about Pazienza at the end of today's session. Scarcely known abroad, Pazienza has achieved enormous notoriety in Italy, with his name being linked to many of the major scandals that have shaken this country over the past five years.

Described as a fixer and social climber who dabbled in espionage, Pazienza is now on trial in Rome in absentia on charges of belonging to a conspiratorial right-wing faction that manipulated Italian military intelligence in the period 1979-81. The group, which was known by the code name Super-S, is alleged to have tampered with investigations into terrorist crimes and to have used the resources of the secret service for their own ends.

Testifying today, Agca disputed reports in the Soviet Bloc media that he had been fed information about his alleged Bulgarian accomplices. He said that implicating the Bulgarian defendants had been his own idea.

"Nobody suggested anything to me," he added. "However, I did meet with Francesco Pazienza, who asked me to collaborate. He had something to do with an embassy and said he was a friend of [Libyan leader] Muammar Qaddafi. He promised me liberty and a French passport. Now he is in an American jail. The person who promised me my freedom cannot even free himself."

Asked when and where the meeting with Pazienza took place, Agca replied: "Between March and April 1982" at the Ascoli Piceno prison, where Agca was sent in July 1981.

After initially insisting that he had acted alone in shooting the pope, Agca began implicating the Bulgarian secret service in the papal plot in May 1982, but not until late October 1982 did he identify the three Bulgarian defendants as his alleged accomplices.

Agca has disputed statements by Giovanni Pandico, an Italian mobster and former inmate of Ascoli Piceno who turned state's evidence, that Italian military intelligence used the mafia to pressure him into alleging a "Bulgarian connection" to the papal plot.

Today's admission by the papal assailant that a third Turkish gunman was in the square came after the judge read out portions of testimony from a Turkish rightist, Yalcin Ozbey, now imprisoned in West Germany on drug-smuggling charges. Interrogated by Italian magistrates in February 1984, Ozbey said that four Turks had taken part in the assassination attempt.

"There was a third man," Agca blurted out suddenly after intense questioning today by Judge Severino Santiapichi.

Pressed to describe the "third man," Agca said he was a member of the right-wing terrorist group known as the Gray Wolves and came from his hometown of Malatya in eastern Turkey. He said he was between 5 feet 7 and 5 feet 8 1/2, thinly built, with black hair, black eyes, and between 25 and 28 years old.

As excitement mounted in the converted gymnasium, Agca was handed a photograph taken at the moment of the shooting, on which he circled a man he called "Akif," evidently a pseudonym. The man in the photo is apparently looking toward Agca who is holding a gun in his right hand pointed toward the pope.

But Agca, despite a plea by Santiapichi to tell the truth about "Akif," refused to identify the figure in the photo by any other name.

Last year, Ozbey identified "Akif" as a left-wing Turkish terrorist named Sedat Sirri Kadem who, he said, had links with the Bulgarian secret services. But he said the man identified by Agca today was 5 feet 11 and had curly blond hair.

Agca has admitted being friendly with Kadem, but insisted today that he was not "Akif." He said he thought that Kadem had been in prison in Turkey at the time of the assassination attempt.

According to Ozbey, whose testimony was made public for the first time today, both he and Agca exploited Kadem's "contacts" with the Bulgarian secret service to escape from Turkey.

Saying he thought Bulgaria had been behind the attack on the pope, "even though this cannot be proved," Ozbey said that the Bulgarian secret service had checked Agca out while he was in Bulgaria in the summer of 1980 following his escape from a Turkish top-security prison.

"As far as I know, he did not have the right qualities for a collaboration. The Bulgarian secret service did not believe in Agca," Ozbey added.

Ozbey, whose own credibility has been challenged by Italian investigating magistrates, said that Agca's real motive in shooting the pope was hunger after fame. He added, however, that Kadem had "ideological reasons" for wanting to shoot the pope because of the upheavals in Poland.

Ozbey, a close Agca associate in Turkey, said his information about the assassination attempt came from a conversation with Celik. Celik, who is being tried in absentia, has not been seen in public since the day of the shooting.