Controlling a main solenoid and two sub-solenoids
Since this sounde like a one-off and therefore a few $ more once shouldn't matter that much, two relays would solve this problem easily.  Get two DPST (minimum) relays with 24 VAC coils.  The existing drive circuitry turns on the relays instead of the valves, which will be indistinguishable to it.  Each relay then turns on its zone valve with one pole and the existing common valve with the other pole.  In other words, the common valve will be separately enabled if either of the new zone valves are on.  All this will happen transparently to the controller if hooked up right.  The controller will only see a additional zone.

This is not cheap or elegant, but just two relays and a few wires and you're done.