Custom cycling of circulator and zone valve...

I've added a radiant zone to my existing boiler (peerless mc series, baseboard, single circulator with 2 taco zone valves). My radiant zone uses a WattsRadiant manifold and Isotherm assembly, T'd off my main loop.

Problem is, I neglected to realize that the main loop circulator is only running when one of the two existing zones is calling (and without it, the radiant zone gets no heat).

I think I know the solution, and its pretty easy mechanically, but I'm not sure how to execute electrically:

If I add a 3rd zone to the boiler (a "bypass zone") with a zone valve, then when the radiant floor calls, I need it to OPEN the bypass zone valve and turn on the circulator ONLY WHEN both other zone valves (1 & 2) are OFF. Similarly, if bypass zone is ON (meaning radiant zone is calling and 1&2 were OFF), and 1 or 2 calls, it would need to SHUT the bypass zone valve.

(This is easier to draw than to explain...).

Anyone know how to do this?? I think I can do this with a couple of SPDT relays, not sure if there is an easier way. (Ie, run each of the existing zone valve's power through a SPDT relay with power to the bypass zone going through the NormClosed contacts of the relay.)

Help! Its winter here, need to solve this asap...

The best way to have done this would have been to run a primary\secondary loop system. The primary loop runs from the boiler feed, back to the return. It should have the circulator installed after the expansion tank and water feed valve (pumping away) All other zones tee off the primary using closely spaced tees (no more than 8" apart) with high demand zones tee'd off first. Control wize, the primary circulator runs whenever any of the other zones call.