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

Nest Thermostat Wiring Colors

Nest learning thermostat wiring diagram Nest thermostat wiring diagram

Thermostat wiring wire heat color diagram air pump nest guide common goodman colors honeywell code heater aruf handler wires labels 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

Nest Learning Thermostat Wiring Diagram
Nest Learning Thermostat Wiring Diagram

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