According to memory:
One AOA is voted out and computer is using two AOAs.
The faulty AOA could be any one (1,2 or 3). If the faulty AOA is number 1 or 2 then you will not be able to tell which one is faulty as both PFDs will have normal display.
you can find out the value of AOA 1 or 2 using Mcdu but cannot find AOA 3 unless transferring IR 3 to side 1 or 2 then using the bird to determine AOA 3.
In the unlikely event of the other two AOAs bieng consistent but erroneous, then there will be a problem. In this case apply OEB (can not remember the number) which tells to switch off 2 ADRs