OLD WESTBURY - A three-vehicle accident snarled traffic for hours on the Long Island Expressway in Old Westbury this afternoon.Police say the crash happened just after 11 a.m. when a dump truck flipped over after colliding with a tractor-trailer and a car.Three of the westbound lanes were closed between exits 39 and 40 while police investigated the accident.Officials say several people sustained injuries, but their severity remains unknown.