Can't figure this 3 wire system out or find anything in postal about it

Wiring A Thermostat For Ac

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Thermostat Wiring Explained

Thermostat furnace hvac transformer explained heating conditioner thermostats wires temp refrigeration evaporator components inside discharge model
Can't figure this 3 wire system out or find anything in postal about it
Can't figure this 3 wire system out or find anything in postal about it

Thermostat Wiring Explained
Thermostat Wiring Explained

Residential Thermostat Wiring
Residential Thermostat Wiring