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What do you have for a boiler? brand and model please? Do you have zone valves (brand?)? or a pump for each zone?
OK, turn down all the thermostats wait 5 minutes then go to each zone valve and push the lever on the end of the valve head/operator to the right. Note if you feel any resistance in the lever and note which one(s) has the resistance. Then tell me what you found
And all your thermostats are turned down?! No resistance at all? If so disconnect the wiring from each thermostat, 1 at a time, then check each valve in turn as before.
So that's telling me the problem is with the thermostats but all 3 failing at the same time is really unusual. Did something happen? a power surge? static discharge at the stats?