You do not need a ZVC...use a SR 504,not a 503....wire the 3 thermostats and circulators to the relay as the instructions show......now in place of the ZVC get a 24volt,40va transformer and wire like this....from the R terminal on the 24 volt side of the transformer go to each R terminal of the thermostat on the zones controlled by the zone valves,from the W terminal on the t-stat back to one yellow wire on the corrosponding zone valve,the other yellows go back to C terminal on the transformer,take one red wire from each zone valve,tie them together and they go to one thermostat terminal on the SR 504,same goes for the other set of reds..(I am assuming that the zone valves are Honeywell v8043 e's........if not, then you will have to figure out which wires are for the zone valve motor,yellows on the Honeywell,and which are the end switch)the circulator for the zone valves gets wired to the zone that has the zone valves on the 504...........XX to TT on your aquastat and there you go!!