The lawyer for Edward Thomas Mann said yesterday his client probably will plead not guilty by reason of insanity to charges that Mann killed three persons and assaulted 23 others during a shooting spree last month at an IBM Corp. office building in Bethesda.

Circuit Court Judge Irma S. Raker set Dec. 6 as a trial date and gave attorney R. Kenneth Mundy 15 days to decide on the insanity plea. Mann, who was indicted by a Montgomery County grand jury Wednesday on 75 counts of murder, assault, and use of a handgun to commit a crime of violence, pleaded not guilty at the arraignment yesterday in Rockville.

"It is more probable than not that the plea of not guilty by reason of insanity will be tendered," Mundy told the judge and assistant state's attorney Michael Mason.

Mundy said he will make the decision after Mann has completed a battery of psychological tests. Until then, Mundy said, his client is satisfied with being held without bond at the Montgomery County Detention Center.

Mann, 38, of Mitchellville, limped into the courtroom on crutches. Mundy said after the hearing that his client tore an Achilles' tendon Thursday while playing basketball with other inmates.

According to police and court records, Mann, former IBM marketing representative, drove his Lincoln Continental through the glass doors of the IBM building off Democracy Boulevard near Montgomery Mall on May 28 and went on a shooting spree throughout the three-story building with an assortment of automatic weapons.

During a 7 1/2-hour siege, 150 rounds were fired by a gunman, killing two IBM employes, fatally wounding a security guard, and wounding six others.