You might be able to hook the one thermostat to each of the three inputs, BUT YOU MUST CHECK OUT THE CIRCUIT TO BE SURE YOU AREN'T CAUSING A PROBLEM.
By understanding the thermostat circuits I believe it should be possible to connect one thermostat so it causes the controller to actuate all three circulators.
If you have a meter you can probably check to see if one of the terminals on each of the thermostat inputs is common to all thermostat inputs. If it is, then you can connect THE OTHER thermostat wire to the other not-common returns so it looks to the controller like there is a thermostat on all of them.
The output relays for the circulators are rated to only 5 Amps so they are probably not large enough to start all of the circulators at the same time.
IF YOU ARE CONFIDENT WITH ELECTRICAL THINGS you can get a 2-pole relay with at least 5-amp rating, with 120 Volt coil, and connect the coil across the circulator terminals that are actuated by the thermostat. Then supply the poles of the relay with 120 Volts and connect the other two circulators to the outputs of the relay. You will also need to connect the other wire of the added circulators to the neutral.
If I were doing it myself I would figure out what I had to do to cause the other two output relays to be actuated when the first circulator comes on, without having to add another relay. However, without being able to look at the wiring I have no way to tell you what to do.
Postscript: I couldn't find the SO503 controller. I found an SR503 controller. http://www.sdsmarket.com/detail.php?id=16328