Extending its losses for the third straight session, the Sensex at the Bombay Stock Exchange tumbled 310 points, or 1.7 percent, to 18,412, today. The Sensex fell on sustained selling by investors, after disappointing quarterly results from IT major, Infosys, poor industrial output data, and weak global markets. The Nifty at the National Stock Exchange slipped 90 points, or 1.6 percent, to 5,526.