The certified Flight Instructor, pilot-rated passenger and non-rated passenger received fatal injuries and the airplane was destroyed at 1013 Eastern time after colliding with a power line, trees and the ground while on an ILS approach to Runway 5 at the Donaldson Center (S.C.) Airport. Instrument conditions prevailed and an IFR flight plan was filed for the cross-country Part 91 flight which originated from Jacksonville, Fla., at about 0800.As the airplane executed the ILS approach, an air traffic controller observed it to descend below the minimum safe altitude of 2500 feet. Attempts by the air traffic controller to reestablish radio contact with the pilot were unsuccessful. At 1021, the local 911 operators received a telephone call, at which time the caller reported the downed airplane 9.4 nm south of the airport. The crash site revealed a damaged power line about 75 feet above the ground tops of four trees that were also damaged.