Three motorcycle bornemen gunned down a 45-year-old farmer in front of his house in Bihar's Rohtas district, police said on Monday.The incident occurred late on Sunday night in Sukahara Dehri village when the gunmen called Gajendra Singh, and pumped two bullets in his chest after he came out of the house, Sub-Divisional Police Officer Raj Kumar said.Singh died on the spot and the attackers fled.The reason behind the murder is not yet known, he said.