The helicopter collided with trees and was substantially damaged at 0530 Eastern time during a cross-country flight from Greer, S.C., that had begin at about 0300 that morning. Instrument conditions prevailed but no flight plan had been filed. The flight instructor and two passengers were fatally injured. A witness reported hearing a helicopter over a residential area and, shortly afterward, heard an explosion. The helicopter was found engulfed in flames. Efforts by the witnesses to extinguish the flames were unsuccessful. No radio communication was received from the pilot prior to the accident.