There is the sentence:
Electrical signals from the three sources are relayed to the brakes and steering control unit (BSCU) and are computed into the nose steering angle.
The question: What are these sources and where they located?
I don't know where your text is from, but the 3 sources are:
The BSCU receives orders from:
‐ The Captain's and the First Officer’s steering hand wheels (orders added algebraically),
‐ The rudder pedals,
‐ The autopilot.
The BSCU transforms these orders into nose wheel steering angle.
(A320 FCOM DSC-32-20-10)