I think I just discovered the problem as to why zone 3 runs with every other zone: it appears to be the master valve.
Even with the Rachio control panel unplugged from power, when I manually twist the solinoid on any zone the zone will not run unless I have zone 3 manually twisted to the “on” position.
My question is how in the world would I make it so that zone 3 doesn’t run with the other zones? It’s overwatering that area tremendously. Was this an original design mistake? Or, perhaps a part is malfunctioning?