For the third time in eight months, Virginia officials are preparing to use the state's electric chair, this time for the execution of 32-year-old Morris Odell Mason, the self-professed "killer for the Eastern Shore," convicted of murder, rape and arson.

Mason's attorney, who contends that Mason is mentally incompetent and should be spared, filed a last-minute appeal today before the Supreme Court.

"Things don't look real good for Morris right now," said attorney J. Lloyd Snook. "I'd have to say the odds are distinctly against him."

An earlier appeal to the U.S. Fourth Circuit Court of Appeals was rejected Friday.

Mason is to die at 11 p.m. Tuesday, but Snook said he hopes Gov. Charles S. Robb will commute the sentence to life imprisonment.

Snook talked today with David McCloud, the governor's chief of staff, and urged clemency. Some civic and religious leaders have urged that Mason's life be spared, but Robb has not indicated he will take any action in the case.

Officials at the State Penitentiary here said they have instituted a modified "lockdown," in which inmates are confined to their cells, to avoid a repeat of the violence that occurred in April during the last execution at the penitentiary.

Then, at least nine guards were injured in a melee that erupted hours before the execution of James Briley of Richmond, whose brother, Linwood, had been executed last October. The Brileys were leaders of a murder and rape crime spree in Richmond and also led the May 1984 escape of six death row inmates at Mecklenburg.

Mason, a former laborer, pleaded guilty in the May 13, 1978, murder of Margaret K. Hand, 71, whose Eastern Shore home was set ablaze after Mason raped her, nailed one of her hands to a chair, and struck her with an ax handle.