Last Friday, the board of Nashville-based Hospital Corp. of America declined to act on the proposal, saying the investor group lacked "demonstrated ability . . . to consummate an acquisition of this magnitude." Three businessmen now say they have withdrawn an offer to pay $47 per share in a $5-billion effort to acquire all outstanding stock in the firm. Dr. Thomas F. Frist Jr., the company's president and chief executive, was notified of the withdrawal in a letter from Charles R. Miller, Richard E. Ragsdale and Richard L. Scott.