As previously mentioned, it happens usually when ADRs disagree. I have been showed a trick which worked in flight ( don't know the technicalities behind it though ) : engage AP 2, reset FAC 1, then engage AP1, reset FAC 2. That can rid you of the CAT 3 SINGLE ONLY on the memo. Of course it goes without saying that if you have CAT 3 SINGLE ONLY memo, you will of course be able to shoot a CAT 3 SINGLE ONLY approach.
Happy landings.