The helicopter went down in a farming field in the city of Chuncheon in the eastern province of Gangwon during a checkout flight, defense and army officials said.
The officials said that all four soldiers aboard the helicopter were initially rescued after the crash, but that three of them died while being treated in hospitals.
They said no casualties on the ground have been reported. The cause of the crash is under investigation,AP reported.
The officials spoke on condition of anonymity, citing department rules.
South Korea maintains more than 600,000 troops under a conscription system to cope with potential aggression from rival North Korea.
The crash occurred amid a heightened standoff with North Korea over its recent nuclear test and long-range rocket launch.