You don't know! Cat 3 is intended for autoland so you have to rely on the kit putting you on the centreline. You will never have sufficient reference to really allow a manual landing. The Cat 3A minima only allows the pilot to ascertain that the aircraft is landing (itself) in the touchdown zone (hopefully). A Cat 3B approach adds the safety of ensuring a rollout on the centreline.
ALL of the above assumes correctly operating (and operated) certified kit in the aircraft and on the airfield.