** Shares of Keystone Realtors KEYS.NS fall nearly 6% to 576 rupees, biggest drop in two months since Aug 5
** Three promoters of co to sell up to 3.65% stake via offer for sale
** Floor price for offer for sale to be at 550 rupees per share, co says
** Offer for sale price at a discount of 10% from Tuesday's close
** Boman Rustom Irani, Percy Sorabji Chowdhry and Chandresh Dinesh Mehta among promoters selling stake, which together have a 76% stake in co as on June 30
** YTD, KEYS down 14%
(Reporting by Komal Salecha)
((Komal@thomsonreuters.com;))