Edward Thomas Mann, charged with murdering three persons and assaulting 23 others during a shootout last May at the Bethesda offices of IBM, has signed a statement pleading guilty to the charges against him.

If the Circuit Court accepts the guilty plea, a judge could sentence Mann to die in Maryland's gas chamber, the penalty that prosecutors are seeking. Montgomery Circuit Judge William C. Miller has reviewed the possibility of the death penalty with Mann, but Mann told his court-appointed attorney yesterday he would like to be executed.

Since he was jailed without bond after the May shootings, Mann has fired one attorney and demanded interviews with the press. The court has tried for six months to settle the questions of whether he is mentally competent to stand trial and whether he should be allowed to be his own lawyer.

A team of eight psychiatrists and psychologists concluded in August that Mann was capable of assisting in his own defense and that he understands the "criminality" of the charges against him.

But one of the doctors, Neil H. Blumberg, later told Assistant State's Attorneys Roger Galvin and Michael Mason that Mann's decision to represent himself in the case had raised new concerns about Mann's mental competency.

At Judge Miller's order, Mann was returned to the Clifton T. Perkins Hospital earlier this month for further mental tests. Although a report on those tests is not yet available, Public Defender J. Theodore Wieseman, Mann's court-appointed attorney, said five of eight doctors found his client mentally incompetent, while three judged him competent.

Hearings on Mann's guilty plea cannot be held until Judge Miller determines whether Mann is mentally competent to stand trial, Wieseman said.

That plea is contained in a one-paragraph statement dated Nov. 18 and signed by Mann in the role of his own attorney. The statement, released yesterday by Wieseman, reads: "The defendant . . . hereby enters a plea of guilty to the offenses charged in the charging document. I make this election knowingly and voluntarily and with full knowledge that I may not be permitted to change this election."

Wieseman said Mann was still anxious to make the plea in court. "He told me today that he wants to plead guilty in court as quickly as possible and that he wants to be executed as quickly as possible," Wieseman said.

Mann, 38, a former IBM employe, drove his 1977 Lincoln Continental through the glass doors of the IBM building off Democracy Boulevard near Montgomery Mall last May 28. During the 7 1/2-hour siege that followed, he allegedly fired about 150 rounds from a variety of weapons before he was taken into custody.