Falling for the third straight session, the Sensex at the Bombay Stock Exchange dropped 246 points, or 1.2 percent, to close below the 21,000-level, at 20,926, today, ahead of key economic data, and amid weak global markets. The Sensex had declined 155 points in the previous two days. The Nifty at the National Stock Exchange fell 71 points, or 1.1 percent, to 6,237.Stock markets in Japan, China, Hong Kong, South Korea and Singapore lost between 0.1 percent and 1.1 percent.