Three people believed to be homeless were found dead outside in the bone-chilling cold Saturday morning in Manhattan and Brooklyn, but no cause of death has been determined, cops said. A 67-year-old man’s body was found outside 528 3rd Ave. in Murray Hill at about 7:45 a.m., cops said. A 64-year-old woman was then found outside at 1112 Remsen Ave. in Canarsie, Brooklyn, at 9:25 a.m. and pronounced dead.  Temperatures plummeted to a low as 10 degrees overnight into Saturday. Around the same time,