You can't get Cat3 Dual back if you are talking about Athr channel 1 fail...
You can get Cat3single ie Cat3a approach capability. With both AP 1+2, Athr channel 1 is working (it has priority). If this fails you will get thrust lock, Athr will drop out and you will just have Cat 2 capability. Simply press AP1 (which will disengage AP1) and press Athr button. You will now regain Athr through channel 2, with AP2 working giving Cat3 single.
The moment you hit AP1 again with a failed Athr channel one, you will go back to thrust lock and Athr will drop out.