The CAT 3 single or CAT 3 dual FMA asked about a few posts ago is an airbus rather than Boeing mode. The cat 3 dual refers to a fail operational situation where should 1 autopilot fail below alert height(100 ft in A320 series) the plane will still auto land safely. This is required for ops with no decision height. With only cat 3 single, it is limited to a cat 3a approach (fail passive)which requires visual reference at 50 feet so if the remaining autopilot fails, the pilot can see this and carry out a go around ( or land with sufficient reference)
It is limited to cat 3 single with some system failures eg engine out.