I thought cat iii meant 3 autopilots hence why the 737 hasn't made it so. We only had 2 autopilots on the 747-200 and the were cat 2. It may be possible that the HUD will allow flight to cat iii as the 3rd indication system but the HUD receives its data from a different source and can conflict with an auto land. Good question and look forward to someone with more knowledge answering it for you