Can you clarify what you are asking?
You should think of it as having 1 "autopilot" or auto flight control system, with 3 processors. If one or more fails, you'll have some annunciations to that effect, but it isn't important which individual processors have failed. They are identical. Nor is it necessary to know which one is active. Typically, computer systems like these will "power on self test" and then select the channel or processor that wasn't used previously.
Think of it like this: If you have a lamp with three light bulbs and one goes out, does it matter which one went out? It just isn't as bright, and you know you need to change a light bulb.