Complex sentence: This feat was also accomplished by future Heaveyweight Champions Cassius Clay known better as Muhammad Ali and Leon Spinks who while serving in the United States Marine Corps won three consecutive United States Amateur Light Heavyweight Championships'74,' 75 and' 76.
Split into simpler sentences: This feat was also accomplished by future Heaveyweight Champions Cassius Clay known better as Muhammad Ali and Leon Spinks. Spinks, while serving in the United States Marine Corps won three consecutive United States Amateur Light Heavyweight Championships'74,' 75 and' 76.