(Reuters) - Elliott Management Corp on Wednesday urged Marathon Petroleum Corp to split into three companies, saying it would boost shareholder value by as much as $40 billion, three years after the activist hedge fund asked the refiner to consider spinning off businesses. Elliott said its call to separate Marathon's retail, refining and midstream assets was prompted by the company's failure to deliver on past promises and "chronic underperformance." The company's shares have fallen 6% this year, compared with a 7.4% gain in the S&P oil and gas refining and marketing index.