Unisys Corp., the computer and defense systems company suffering from a crippling debt load amid slowdowns in its key businesses, reported a $357 million loss for the third quarter and said it expects more red ink.

At the same time, Michael Blumenthal, the former Treasury secretary who guided the company's birth four years ago through the merger of Burroughs and Sperry Corps., severed his last tie with the lumbering giant and resigned as chairman of the board.At the time of the merger, Unisys was the nation's second-largest computer company. But it has been shrinking while the competition has been growing and it now ranks third.

Blumenthal, a limited partner in the investment banking firm Lazard Freres & Co., said he would leave Nov. 1 due to "increasing international obligations."

Unisys, based in Blue Bell, Pa., said it lost $356.8 million in the third quarter, down from the loss of $648.2 million in the same period last year. Revenues during the period were $2.4 billion, vs. $2.35 billion in the third quarter of 1989.