Ayuh,.... Post some pictures of yer near boiler plumbin', 'n where the zones are split off,....
I'm sure you can drain the system, cut the pipin', 'n add valvin' to isolate that zone, or each of the 3 zones,....
When I build my systems, I put valves 'bout everywhere,....
I can isolate 'bout any component of my system,....
That way, if there's a problem, ya just turn a couple of valves, fix it, 'n reopen the valves,...
Lots less bleedin' the bubbles outa the system,....