Stuart L. Kreiner, the 16-year-old charged with last week's stabbing deaths of three young girls in his Glen Burnie, Md., neighborhood, today pleaded innocent by reason of insanity and was ordered to undergo psychiatric testing at the state mental hospital.

A few hours after an Anne Arundel County grand jury indicted Kreiner on three separate counts of first-degree murder the youth's attorney, J. Edward Davis, entered three separate pleas on his behalf: innocent, innocent by reason of insanity, and innocent because Kreiner was insane at the time of the slayings.

The second plea indicates that the youth may still be insane and thus unable to aid his attorneys in preparing his defense; the third indicates simply he was insane at the time of the slayings.

Kreiner was led into Circuit Court Judge Raymond G. Thieme Jr.'s courtroom under heavy guard this afternoon, and, after taking his place at the defense table, stood silently throughout the proceeding, his head down, his knuckles resting gently on the edge of the table.

The indictments issued earlier in the day charge the youth with stabbing to death two sisters, 10-year-old Deborah Hogan and 8-year-old Theresa Hogan, and Ann Brzeszkiewicz, 8.

Kreiner is scheduled to be taken to Clifton T. Perkins state mental hospital on Tuesday morning to begin a two-to-four month stay.

Until then the youth is being held without bond in an isolation cell at the Ann Arundel County Detention Center here. Deputy State's Attorney David Cuttler said today that Kreiner also will be kept in isolation at the hospital.

A trial date probably will be set in the case some six to nine weeks after a psychiatric report evaluating Kreiner is released. Cuttler indicated. Depending on the conclusions of this report, a number of different outcomes are possible.

If the report indicates that Kreiner is insane and unable to aid in his own legal defense, he can be confined at Perkins either until he is sane and able to stand trial or until he has been at that hospital for 10 years.

At the end of that period, Kreiner could be released if no further legal attempts were made to keep him in custody.

If the report indicates that he was insane at the time the girls were killed, but that he has since regained his sanity.Kreiner could face trial on the three murder charges.

If, at the conclusion of such a trial, the youth was found to be insane at the time of the slayings, he could be sent back to Clifton T. Perkins hospital, to be incarcerated until the doctors there determine that he is no longer a danger to himself or to society.

If the psychiatrists find Kreiner sane, it is likely that the youth will go to trial as soon as the court schedule will permit.

In his request for an immediate psychiatric evaluation of Kreiner, defense attorney Davis specifically asked that Dr. Jonas R. Rappeport, the forensic psychiatrist for the Baltimore City criminal courts, be included among the examining physicians. Judge Thieme granted this request, along with the request for immediate treatment.