My understanding is that "3C" will have nothing to do with the aircraft - it is already landing - it couldn't care less what the visibility is. The move to "3C" will be dictated by ground facilities, particularly the problems associated with seeing where the hell you are after landing and having cleared the runway. FOr many autoland operations, there is no requirement to see anything prior to touchdown, hence the "Alert Height" - (the Frogs do have that requirement hence a "Decision Height"). Past experience has shown that the problems begin after thinking you have cleared the runway. Ask United about pilot disorientation after landing in low vis. In Cat 3C, SMIGS would be useless - you wouldn't be able to see a foot in front of your face.