Falling for the third straight session, the Sensex at the Bombay Stock Exchange dropped 51 points, or 0.3 percent, to 18,086, on Wednesday, on sustained selling by funds, amid interest rate concerns and lower-than-expected corporate earnings. The Sensex had lost almost 400 points in the previous two trading sessions. The Nifty at the National Stock Exchange shed 18 points, or 0.3 percent, to 5,421.But stock markets in Japan, China, Hong Kong, Singapore and South Korea gained between 0.2 percent and 1.6 percent.