Soon after accused slayer Edward Thomas Mann quit his job at IBM three years ago, he began believing that he was a victim of a "global conspiracy," which included his wife and son, that wiretapped his house and forced him to become involved with male and female prostitutes, a psychiatrist testified yesterday.

Neil Blumberg, a psychiatrist at Clifton T. Perkins State Hospital, described at least 35 instances in which Mann was convinced that nearly everyone with whom he came in contact was trying to destroy him. Mann was moved to the hospital in Jessup last week after he attempted suicide at the Montgomery County Detention Center in Rockville.

Mann, 38, is charged with killing three people and assaulting 23 others in a siege last May on the Bethesda offices of IBM.

A hearing began Monday in Montgomery County Circuit Court to determine if Mann is mentally competent to stand trial. Mann has signed a guilty plea and has asked to represent himself, but Judge William C. Miller said he will not rule on the plea until the competency hearing concludes later this week.

Blumberg was the second of three psychiatrists called by Mann's public defenders to testify that he has an extreme case of paranoia and is not capable of representing himself.

Blumberg characterized Mann as a tortured person who became increasingly withdrawn and suspicious during the last three years. At one point, Blumberg said, Mann began placing hairs across doors in his house and checking his shag carpet to see whether anyone had entered his house while he was out.

Blumberg also said Mann canceled a newspaper subscription when he became convinced that persons involved in the conspiracy were planting subliminal messages in the paper in an effort to manipulate him. Blumberg also said Mann desribed friends who had "led him" to pick up male and female prostitutes in the District as part of an effort "morally and ethically break him."

Mann abandoned the prostitutes, Blumberg said, when he became convinced that even they were involved in the conspiracy. All the prostitutes, Blumberg said, told Mann they wanted to be data processors or computer programmers. Mann had been employed as a computer technician.

"What started as a small spiral of things gradually expanded . . . to the point where the world for him and the people he could go out and see and feel safe with gradually narrowed," Blumberg said. "It was a spiral . . . that he eventually thought would break him."

The turning point, Blumberg said, came in the summer of 1980 when Mann asked his wife to leave because he suspected she was involved. At that point, he bought his first of three guns and began to refuse to answer the door or the phone if the caller first did not "ring their signal."

Under cross-examination, assistant state's attorney Michael Mason tried to refute Blumberg's testimony by indicating that despite Mann's narrowing of his social ties, he continued to go out and see some friends and play cards with his wife's family.