heating zone stays hot even when the thermostat isn't calling
You need a check valve on all three return lines from the 3rd floor to prevent flow from the second floor loop going through 3rd floor radiators.  Follow your diagram from the circulator through the first 2nd floor radiator to the Tee.  From the tee water can go "north" to feed the third floor radiator and return to the loop through another 3rd floor radiator from the common manifold (path of least resistance).  A check valve is not electric and allows flow in only one direction.