Rising for the third straight session, the Sensex at theBombay Stock Exchange gained 104 points, or 0.6 per cent, at 17,937, today, on hopes of strong quarterly corporate earnings, good progress of monsoon rains, and positive Asian markets.The Sensex also crossed the 18,000-point level during the day, but later eased on profit-booking. The Nifty at the National Stock Exchange rose 31 points, or 0.6 percent, at 5,383. Stock markets in China, Hong Kong, South Korea and Singapore rose between 0.3 percent and 0.8 percent.