A trio of robbers targeted monks at gunpoint at a Buddhist temple in Brooklyn on Sunday, police and sources said. The heartless crooks broke in through the back door of Watt Samakki Buddhist Temple on Rugby Road near Caton Ave. in Flatbush just after 2:25 p.m., cops said. Once inside, one of them pulled a gun on four monks and demanded they turn over cash and property. The men took off on foot northbound on Rugby Road and are still being sought, police said. Police were still investigating the burglary Sunday evening and determining how much cash and property the robbers made off with. A neigh…