Put simply, those modes will only come up if everything is working properly. Lets assume you are doing a Cat 3b approach ( don't know anyone who does Cat 3c). You need 75m forward viz. If you lose a signal higher up you get an autoland warning and you go around. We have plenty of warnings to let us know if something has failed, usually something flashing on the FD!! If we don't get Land mode we go-around if not visual. We call "alert height" at 100' which is our final check. Depends on the problem really. We can also go-around from any point, even if the wheels have touched.