As per your examples how to take the RER B from CDG terminal 1 or 2 and the shuttle train will go from either terminal 1 or 2 to terminal 3. We will arrive directly at CDG terminal 3, so I assume that we will go down the escalator and take the RER B directly from the platform?
CDG Terminal 3 is within 5 minutes walking distance to the CDG Airport Train station that serves both CDG Terminal 3 and CDG Terminal 1. This train station is called Aeroport CDG 1 and is located inside the Roissypole building in between both CDG Terminal 1 and CDG Terminal 3.
After purchasing your RER tickets at the only ticket window inside Aeroport CDG 1 / Roissypole, you will pass through fare gates using your RER tickets and descend down the escalators to the RER B train platform, take care to go in direction of Paris.