AFAIK, the following is what they are updating:
✔ Uncompressed HDMI Output Support
When shooting video, HDMI Output makes possible the recording of high-definition uncompressed video data (YCbCr 4:2:2, 8 bit) from the EOS 5D Mark III to an external recorder via the camera’s HDMI terminal.
✔ This enhanced capability facilitates the editing of video data with minimal image degradation for greater on-site workflow efficiency during motion picture and video productions.
✔ Video being captured can be displayed on an external monitor, enabling real-time, on-site monitoring of high-definition video during shooting.
✔ Improved AF Functionality – Even when the EOS 5D Mark III is equipped with an extender and lens making possible a maximum aperture of f/8, the firmware update supports AF employing the camera’s central cross-type points (currently compatible with maximum apertures up to f/5.6).
✔ This firmware update will allow users to take advantage of AF when shooting distant subjects, benefitting sports and nature photographers, particularly when using telephoto lenses.
Word on the street is that "Canon 5D Mark III firmware update will also come packed with several bug fixes. One of them will allow the camera to focus faster when using the Speedlight AF Assist Beam, while other details remain unknown for now".