Three men arrested Wednesday in connection with a bizarre plot to steal and sell a nuclear submarine may have believed their potential buyer was not a foreign power, but an organized crime chieftain.

Federal sources said yesterday that an FBI undercover agent in St. Louise posed as a money man for hire and train a 12-man crew to board the USS Trepang in New London Conn., harbor, kill the crew and sail the sub out into the Altantic Ocean to meet the new owner.

One of the suspects deduced from conversations with the FBI undercover agent "that he was dealing with the Mafia," a source said.

That twist is just the latest in what Defense and Justice department officials have called a wild tale that seems a cross between a James Bond and a Woodie Allan movie.

But when the alleged plotters - who included one man who had served on the targeted sub - asked the undercover agent for $300,000 to finance the plan, federal authorities in St. Louis decided to make arrests, sources said.

"I don't know if they could have carried it out." Roy B. Klager Jr. special agent in charge of the FBI's St. Louis office, told the Associated Press. "But the government is not in a position to take that kind of risk. We were not in a position to discount it as too outlandish."

Sources said the FBI was told of the alleged plot by a St. Louis man who had been approached to act as a possible broker to sell the stolen sub.

The undercover agent then was brought in to act as a middleman for the unidentified party interested in buying the vessel.

Arrested were James W. Hargrove, 26, of Ovid. New Y., and Edward J. Mendenhall, 24, and Kurtis J. Schmidt, 22, of St. Louis.

Cosgrove was reported to be a Navy veteran who was once assigned to the Trepang.

Members of the 100-man Trepang crew told the Associated Press yesterday that they were not frightened, but amused by the alleged plot. "I think it's funny as hell," one sailor said.

"There's no way they could have moved the submarine without a full crew, let alone fire a missile," said another.

In making the arrests, the FBI had said one plan for covering the escape of the plotters was to fire a nuclear missile at an Eastern Seaboard city to create a diversion. The Trepang's missiles are nuclear-tipped antisumarine torpedoes.