Wouldn't be my choice. The 1D MIII is geared toward sports and fast action shooting, street/photojournalism and is suitable for all around use. It's not just the difference in FPS but focus speed as well as other custom functions on the ID MIII.
The 5D was geared toward studio, portrait, landscape etc. It can be used for sports as long as you understand the limitations it will impose on you and can live with them. Just as the 1D MIII can be used for landscape, studio, portrait.