At least three of the jurors who found John W. Hinckley Jr. not guilty by reason of insanity had relatives who were once confined to a mental hospital, according to transcripts of interviews conducted before Hinckley's trial.

The transcripts, unsealed by Judge Barrington D. Parker yesterday, show that a fourth juror had an older sister who was treated for a nervous breakdown and that a fifth juror had received brief psychiatric treatment after a car accident more than 20 years ago.

At least two of the jurors indicated that a person who committed a crime like Hinckley's could be expected to have some mental problem, but both said they would have an open mind about his case.

Hinckley, meanwhile, spent his first day in St. Elizabeths Hospital for the mentally ill yesterday, in a top-floor, maximum-security ward in which he mixed freely with about 25 other patients, was assigned his own room, and was subjected to the standard "ward discipline" of routine cleanup and other chores, according to a hospital spokesman.

"The idea is to get him to relate acceptably to his society, which initially is this group on the ward, and eventually to all society," said D.K. Coyle, hospital spokesman. Coyle said that while the ward staff had been increased because of fears for Hinckley's safety, he was not being isolated because, from a therapeutic point of view, "communicating with others will be pretty important to him. It might open up a whole new life to him."

The would-be presidential assassin was given a thorough physical exam yesterday while a task force consisting of a psychiatrist, psychologist, social worker and other specialists prepared to begin a detailed examination of his psyche. In 50 days, their report will be presented to Parker in the first of a series of hearings that could be held as often as every six months to determine if Hinckley should be released because he is no longer a danger to himself or the community.

The jury's verdict Monday night that Hinckley was legally insane when he wounded Reagan and three others has prompted a flood of public criticism and demands for changes in the insanity defense. The transcripts released yesterday reveal that Parker asked some of the jurors whether they would be upset if their verdict was unpopular.

"I feel that I wouldn't be upset . . . . I wouldn't really care about what they, you know, have to say," juror Belinda A. Drake told Parker during the questioning in late April. Other jurors also said they would not be affected by public reaction to their verdict.

After the jury's decision was made public, jurors Nathalia L. Brown, 30, and Maryland T. Copelin, 50, told reporters they held out for a guilty verdict but eventually gave in because they felt tired and pressured by the other jurors.

Before the trial, when Brown was asked if there was any reason why she should not be included the pool of jurors, she said, "I feel that I'm a citizen of the United States and I would like to serve . . . and help society if that is what it is, you know." In the transcript, Brown, who works for Potomac Electric Power Co., told Parker she had twice been victim of an armed holdup.

When Copelin was asked about her potential service on the Hinckley jury, she said, "I believe I could render a fair verdict." Copelin works for the D.C. public school system. Her husband retired Jan. 8, 1981, from the White House greenhouse, where he had worked for 33 years.

Juror Woodrow Johnson, 48, a garage attendant, when asked whether he had an opinion about Hinckley's mental condition on the day he attacked the president, said:

" . . . I just believe that if a person runs out in front--if he runs out amongst a lot of men and fires, and all of them have a lot of guns and he has taken the chance to get killed himself, I just believe that there is something wrong." But Johnson, asked again about the Hinckley case, added, "I have not made up my mind."

Juror Merryanna L. Swartz, 31, who has an academic background in psychology, when asked that same question said, " . . . Anyone who attempts to shoot a president, or perhaps anyone, you might think would be insane." When Parker asked her if she could put aside her "tentative opinion" and consider the evidence in Hinckley's case with an open mind, Swartz said, "Yes, I think I could."

Hinckley's father, John W. Hinckley Sr., told ABC yesterday that he is "sorry" about the uproar over the verdict in his son's case, "but we and the jury felt it was the proper verdict."

"If the public could have sat through the trial and heard all the evidence that we heard, and the jury heard, I'm sure they would have a different opinion than they have," Hinckley said.

The transcripts show that Parker asked a broad range of questions designed to find out whether the potential jurors had formed opinions on the Hinckley case or whether there were events in their personal backgrounds that might impair their ability to reach a fair and impartial verdict.

For example, he asked all the jurors whether anyone in their families had ever suffered from mental illness. He asked them all what they had read or heard about the Hinckley case and questioned whether any regarded the insanity defense as a "sham defense." The jurors all said they did not consider the insanity defense a sham.

The judge asked the jurors whether they would be prejudiced against Hinckley if they heard evidence--eventually introduced at the trial--that he had made antiblack and anti-Semitic statements. All said they would not be prejudiced against Hinckley for such remarks.

As Hinckley moved into the routine of the John Howard Pavilion at St. Elizabeths yesterday, an official who is intimately familiar with the workings of the facility predicted it would be a long time before Hinckley was released.

This man is going to fall into a group . . . that is not going to be released any time soon," said Harry Fulton, chief of the District of Columbia Public Defender Service's mental health division. "He could expect to stay there, I would measure it in decades."

Fulton, who has worked with the criminally insane in the pavilion for years, said judges, who must approve any releases from the pavilion, are "very conservative" about letting potentially dangerous patients out, even for a few hours at a time with family members or others.

He said they are cautious because prosecutors actively seek to prevent releases and psychiatrists are often unable to predict that a patient will not be violent in the future.

Medication to combat severe depression, violent outbursts and other symptoms of mental illness forms a major part of the treatment in the Howard Pavilion, according to Fulton, who praised the pavilion staff's ability to conduct initial psychological evaluations.

"What they do best is to try and get a fix on the man, to see where he is mentally," he said, adding that after evaluation patients are treated through group therapy and some individual therapy. " 'Milieu therapy' is the biggest form--just being in a psychiatric milieu helps them grasp reality," he said.