|If you don't want the computers in B&C to be able to access each other directly, then you should move the printer and attach it to A. That would make it accessible to B&C without given B access to C, and C access to B.|
If you're not concerned about B&C accessing each other then having separate subnets behind separate routers is moot and you should just buy a switch, plug it into A and connect everybody to it.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.