What are the three sources that are relayed to the brakes and steering control unit of an A320?
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)