Cmdr. Donal M. Billig was sentenced by a Navy jury yesterday to four years in prison and dismissal from the service after his conviction last week in the deaths of three patients while serving as chief heart surgeon at Bethesda Naval Hospital.

The 55-year-old Billig, who was convicted by the same jury, was taken into custody by an officer of the Naval Investigative Service immediately after the sentence was read by Rear Adm. Harry S. Quast, president of the jury. The panel deliberated 1 1/2 hours before handing down the sentence.

The dismissal, which calls for immediate loss of pay and allowances, is the equivalent of a dishonorable discharge. It was only the second time that a Navy doctor has been sentenced by the service on involuntary manslaughter charges.

The sentencing followed a soft-spoken plea from Billig, who stood before the jury and asked for leniency, saying that he had suffered enough from the yearlong investigation by the Navy and the hospital. "I've already lost what has occupied my life and energy for the past 38 years," Billig said. "To me, that is a punishment . . . with a severity I don't think I can ever describe to you."

Navy spokesmen said Billig would spend the night at the Marine base at Quantico and then would be transferred to the U.S. Disciplinary Barracks at Fort Leavenworth, Kan.

Under military law, Billig's sentence is to be reviewed by the naval authority who convened the court-martial, Rear Adm. R.C. Austin, chief of naval technical training, and the Navy-Marine Corps Court of Military Review. If Billig requests, the sentence could be appealed to the Court of Military Appeals and, ultimately, the U.S. Supreme Court.

He will be eligible for parole review after 16 months, one-third of his sentence. He has been in the Navy since 1982.

Billig was found guilty last week of two counts of involuntary manslaughter, one count of negligent homicide and 18 counts of dereliction of duty in connection with operations he performed in 1983 and 1984 at the Navy's premier medical institution. He was acquitted on two counts of involuntary manslaughter and six counts of dereliction of duty. Under those convictions the doctor could have faced a prison sentence of 11 1/2 years.

Billig left the courtroom silently after the sentencing, walking with his wife Bonnie and his daughter, Andrea, of Long Branch, N.J. He and his attorneys then conferred in an office on the third floor of the courthouse at the Washington Navy Yard before he left with naval agents for Quantico.

The sentence ended a two-month court-martial and followed hours of emotional testimony from relatives of patients Billig was charged with killing. There also were conflicting statements from Billig's present and former wives about his character, criticism of the press and Billig by the commanding officer of the Bethesda hospital over the bad publicity the events had generated and praise from a former patient who said that in 1972, President Nixon had described Billig to him as "the best."

The prosecution told the jury yesterday that Billig no longer deserved to wear the Navy uniform and should be sent to prison for at least five years and nine months -- half the maximum sentence.

"He will never believe he is wrong unless you make him believe it," said prosecutor Col. Gerald Miller. Billig's defense attorney, Lt. Cmdr. Stephen Baker, in turn asked the jury to show compassion for a man "whose lengthy career as a doctor is gone" and whose ability to support his family "has been crippled."

"The man awaiting your sentence is already a broken man," Baker said. "What good would it do to put this 55-year-old man in jail?" The arguments of the two attorneys came after a morning of appearances by people whose lives were changed after contact with the heart surgeon.

Kiyomi and Michael Estep, widow and son of retired Petty Officer Joe Estep; Mary Kas and Marilyn Tangusso, widow and daughter of retired Lt. Col. John Kas Jr.; and Lily Grubb, widow of retired Maj. William Frank Grubb, told the jury about life without their spouses and fathers.

"I can still see how he looked" after his operation, said Tangusso, whose father's chest was so swollen after surgery by Billig that his chest couldn't be closed. "I couldn't recognize him. And a lot of times if I'm home alone, that picture comes into my mind."

The commanding officer of Bethesda also took the stand, to complain that publicity about Billig had hurt the reputation of the hospital and its surgeons.

"The hospital finds itself on trial as well as Dr. Billig," said Capt. Stephen Amis, who quoted stories about naval medicine that ran in The Washington Post, the New York Times and Time magazine. "The reputation of the hospital is on trial as well as Dr. Billig. The press has placed the hospital on trial."

Also testifying was Marilyn Billig, Billig's former wife and an artist who lives in Washington, who said she felt "a responsibility to come forward." She described her former husband as "not a truthful person" in their marriage and in his professional life. They were divorced in 1978.

His present wife, Bonnie, an occupational health nurse for the Navy, testified in contrast that her husband was "honest, forthright and a human being of all human beings."

"I'm so proud to be married to Cmdr. Billig," she said. They have been married for 13 months.

Thomas G. Dunn, mayor of Elizabeth, N.J., also typified Billig as a "compassionate, humane doctor." He chose Billig, who then was a civilian doctor in Philadelphia, for his heart surgery in 1972, he said.

Dunn, a Democrat working at the time for the reelection of Richard Nixon, said he received a call from the president shortly before his operation.

"President Nixon assured me I had the best in the business," he told the jury.