Rising for the third straight session, the Sensex at the Bombay Stock Exchange gained 46 points, or 0.2 percent, to 18,870, despite weak global markets, today. The Sensex has gained over 290 points in the past three trading days. The Nifty at the National Stock Exchange added 12 points, or 0.2 percent, to close at a more than 17-month high of 5,731.But stock markets in Japan, Hong Kong and Singapore ended weak.