U.S. District Court Judge John J. Sirica yesterday cut by half the original prison sentences he imposed on the three top figures convicted in the Watergate scandal - former Attorney General John N. Mitchell and former top White House aides H.R. Haldeman and John D. Ehrlichman.

Sirica's reduction of the sentences for the three men to 1-to-4 years probably will be his last official Watergate-related act since he assigned the original Watergate break-in case to himself more than five years ago. He ruled from the bench, without explanation for his reasoning, after an unusual hearing in which the defendants' recently tape-recorded statements of contrition were played in open court and their attorneys begged for leniency for their clients.

The ruling by the judge leaves uncertain the exact dates of the release from prison for the three defendants, the only Watergate figures still in jail. Ehrlichman, who has already served 11 months in prison and therefore will be eligible for parole in the Watergate cover-up case by the end of this month, probably will be the first to be considered for release.

However, Ehrlichman also is serving a concurrent 20-month-to-five-year prison sentence imposed by U.S. District Court Judge Gerhand A. Gesell for his conviction in the White House "plumbers" case. Gesell has not yet ruled on a motion to reduce that sentence, and if it is not reduced Ehrlichman would still have to complete a 20-month minimum for that conviction.

Haldeman and Mitchell have been in prison since June 22, when their appeals were completed. They will not be eligible for parole until next June 22.

Although each defendant on tape and each attorney in person made strongly personal pleas for release from prison, the common threads that ran through all the arguments were that of remorse stated by the defendants and the attorneys' claims that each defendant was broken financially and otherwise by his plight.

For former Attorney General Mitchell, defense attorney William Hundley said, the major factors in favor of his early release are a hip condition that requires major surgery, his "true remorse" over his role in Watergate, and the need of his teenage daughter to have her father at home.

"Even one day in jail for a former Attorney General of the United States is a very, very severe sentence," Hundley said in asking that Mitchell's prison term at Maxwell Air Force Base in Alabama be reduced to time served.

Sirica had asked Herbert Vogt, deputy chief of the U.S. Probation office here, to visit the defendants in prison to ascertain their current attitudes and to tape those conversations. Vogt took the witness stand yesterday and played those tapes for the court, an unusual procedure that eliminated the need for the defendants to be brought from their prisons to the courtroom for the hearing.

Mitchell, 64, his voice firm but somewhat quieter than normal, said he was "truly sorry" for his actions and that no set of circumstances would ever again "lead me to commit such deeds." He said he has "accepted the outcome" of the judicial process, and that his reflections in prison had "led me to considerable remorse and distress."

Ehrlichman's attorney, Stuart Stiller, read a portion of a letter his client had written to the judge in June, in which the 52-year-old former White House aide proclaimed, "I am guilty in law and in fact and I have learned to accept this."

In one of the few mentions of former President Nixon in yesterday's hearing, Ehrlichman said he allowed himself to be "abused" by the former chief executive.

In his 25-minute, sometimes emotional tape-recorded statement, Ehrlichman warned presidential advisers to watch our for "red flags" that indicate their conscience is being taken over by that of their supervisor. "I abdicated my moral judgments and turned them over to someone else," Ehrlichman said in the interview taped at the Safford, Ariz., prison camp.

Haldeman's tape statement, in which he stated "very real remorse . . . I do totally accept responsibility for my action," was delivered in a monotone familiar to those who watched him testify in various hearings.

Haldeman, 50, said he was not sure what he would do upon release from the Lompoc, Calif., federal prison, but that he was sure he would be able to make a "constructive contribution to soceity to counterbalance the destructive contribution" he made previously.

"I have strong feelings of responsibility that whatever wrong was done will never be done again by me," he said.

All three defendants were originally sentenced to 30-months to 8 years in prison after their convictions on Watergate cover-up charges.