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

Home Ac Thermostat Wiring

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Goodman furnace thermostat wiring diagram Can't figure this 3 wire system out or find anything in postal about it

Goodman Furnace Thermostat Wiring Diagram - 18 Goodman Ac Wiring

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

Goodman Furnace Thermostat Wiring Diagram - 18 Goodman Ac Wiring
Goodman Furnace Thermostat Wiring Diagram - 18 Goodman Ac Wiring