Falling for the third straight session, the Sensex at the Bombay Stock Exchange dropped a marginal 13 points, or 0.1 percent, to 18,670, today. The Sensex had lost almost 220 points in the previous two trading sessions. The Nifty at the National Stock Exchange fell 3 points, to 5,684.Stock markets in Japan, China, Hong Kong, South Korea and Singapore ended mixed.