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There needs to be a common wire connected to C on the thermostat, tell me if your Nest models have that C terminal or not, do not wire it yet. I still need more data, but we can begin there.
Hello again, there are several dozen different wiring configurations, we need to work from wiring diagrams. verbally so to speak will not work. It can be a bit more complicated than you might imagine,
If you can make a pencil drawing of the three thermostats showing how they are all wired and what each controls, of if all 3 go to a zone control box, and a the brand and model of the zone control box, that will help immensely.
You are welcome
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I will be back in the morning if you want to send me a drawing etc, we can go from there in that case.