Why does my first floor thermostat also make my third floor baseboards turn on?
There's a chance that a backflow-preventer valve is stuck open, allowing hot water from Zone-One piping to flow &quot;backwards&quot; into Zone-3 piping.  See if you can find these valves- they likely are close to your furnace. They are probably green, with a directional arrow printed into the metal housing (and have only one inlet and one outlet).
Also,  test by &quot;feel&quot; : when the circulator pump for Zone-1 comes on, does the circulator for Zone 3 turn on? .   Or, simply close that valve (knob just above the circulator) for Zone3 and see if that stops the warming. If so, it's almost certainly a jammed backflow preventer.
I have successfully &quot;tapped and tweaked&quot; backflow preventers to unstick them.  All that's inside one is a weighted plunger which, under reverse pressure, drops down and seals the valve opening (under positive pressure, it slides upwards to let water flow in the desire direction).