One of the Cuban exiles charged with assassinating former Chilean ambassador Orlando Letelier boasted in prison of plotting to blow up Russian ships in American harbors and taking part in three attempts to kill Fidel Castro, a jailmate testified yesterday.

Sherman Kaminsky, a convicted extortionist, told U.S. District Court Judge Barrington D. Parker that he and Alvin Ross Diaz talked frequently last year when they both were imprisoned in New York City, and that he soon "realized this was a very dangerous man."

Kaminsky, a one-time member of the Haganah, the Zionist military organization defending Palestine before the creation of Israel, said Ross had told him of a plot he envisioned to take "motorboats load them with explosives and then by remote control (send the boats) to blow up Russian ships in American harbors."

In addition, Kaminsky said Ross had bragged to him that he once personally fired a bazooka at a motorcade that included a car carrying Castro, but the shell hit a car just behind Castro's and killed "some people," not the Cuban leader.

Whether Ross ever took part in such an assassination attempts could not be determined yesterday, but sources close to Ross said he has not been in Cuba since participating in the abortive 1961 Bay of Pigs invasion.

Whether Ross' alleged plot to blow up Russian ships was serious or not, Kaminsky said he had taken it seriously. He said he had told his lawyer to tell the Central Intelligence Agency about the Ross plan because he thought Ross was someone who "could create an international incident."

What is undisputed is that Letelier and a colleague at the Institute for Policy Studies, Ronni K. Moffitt, were killed when a remote-control bomb blew up Letelier's car as they were driving along Washington's Embassy Row on the morning of Sept. 21, 1976.

One of Ross' codefendants, Guillermo Novo Sampol, like Ross a leader of the staunchly anti-Castro Cuban Nationalist Movement, was convicted five years ago of plotting to blow up a Cuban ship anchored in Montreal. Five days before the Letelier assassination, another anti-Castro group called Omega 7 claimed responsibility for bombing a Soviet freighter docked in Port Elizabeth, N.J.

After hearing of Ross' alleged plans and exploits, Kaminsky said he decided that "being a witness in (the Letelier) case was something I wanted to do because Alvin Ross Diaz stands fo reverything that I dislike in a human being."

Parker kept the seven-woman, five-man jury out of the courtroom as he heard Kaminsky's testimony and that of Antonio Polytarides, a former Reston man recently paroled after serving less than two years of a five-year term for illegally selling 100 submachine guns to Irag's United Nations mission in New York.

Polytarides, also imprisoned with Ross and Novo in New York last year, testified that Novo told him that "our group," meaning the Cuban Nationalist Movement, "was responsible" for the Letelier assassination.

Then, late last year, Polytarides said that Novo "was very angry" when he came to him one day because Cuban Nationalists, Novo allegedly said, had "been betrayed by certain people in the (Letelier) case, but they will pay them back."

Polytarides said that Novo wanted to purchase 160 machine guns, 20 pounds of explosives and 200 fragmentation hand grenades from him. But Polytarides said the transaction did not occur.

Parker heard the testimony without the jury in order to determine whether Kaminsky and Polytarides can tell their stories to the jury. Defense lawyers for Ross, Novo and the third defendant in the case, Novo's brother Ignacio, have claimed that Kaminsky and Polytarides were planted by the government to try to elicit damaging information for use in the Letelier case.

But the government's prosecutors, Assistant U.S. Attorneys Eugene M. Propper and E. Lawrence Barcella Jr., argued that most of the damaging statements that Ross and Novo made to Kaminsky and Polytarides, respectively, were unsolicited and occurred before the government knew the two Cubans were allegedly making jailhouse admissions.

"I never initiated any conversation with Mr. Ross," Kaminsky testified. "Mr. Ross will talk and talk and talk as long as you will listen. One time, I literally had to run to get away from him."

The 50-year-old Kaminsky, awaiting sentencing on two of four extortion charges to which he has pleaded guilty, said that the government had never solicited his help in drawing comments from Ross about the Letelier case.

Polytarides, 33, said that Guillermo Novo had admitted the Cuban Nationalists' participation in the Letelier killing the first time he met him. But he said a Customs Service agent later asked him to try to learn more from Guillermo Novo about two fugitives -- apparently a reference to Virgilio Paz Romero and Jose Dionisio Suarez Edquivel, Cuban exiles accused in the Letelier slaying who have not been arrested.

Parker said he would hear more arguments on the admissiblity of the testimony on Monday and then make a ruling.