At about 11:38 CDT, a TWA DC-9 had its landing gear separate on rollout at Nashville International Airport. Three passengers were treated for minor injuries. The First Officer said he turned final and his sink rate was 600 to 800 fpm. The airplane touched down hard on the right main landing gear, which surprised him and the PIC. The PIC immediately got on the flight controls with him as the airplane bounced. On the second impact he felt a vibration, and it became evident that they had a directional control problem. The left wing started to settle and made contact with the ground. The captain said that, right before touchdown, he knew the first officer was not going to make a good landing. The airplane touched down hard, but he did not expect a hard landing of that magnitude.