The OP is not talking about 3 ADRs failing which is highly unlikely, they are talking about the inputs to all 3 ADRs being unreliable or being unable to identify which one or two is still reliable which is entirely possible. Airbus doesn't specify which one to leave on so take your pick, leaving No. 3 on would avoid being distracted by potentially unreliable data on the two PFDs.
Hopefully if and when it happens you will be flying an aircraft with the GPS back up speed scale which simplifies the task hugely.
Last edited by Max Angle; 24th Apr 2017 at 18:47.