a/c thermostat

One wire is power, the second is the compressor, and the third is the rear fan. The idea is when you turn on the switch, the fan in the back runs all the time and the compressor is cycled from the thermostat. Some switches go bad on the fan side and they just set the fan to run with the compressor. Your switch might be bad and need replaced. They are not very expensive. SoBills wiring diagrams will have the correct locations of wires on the switch.

thanks, I did change the thermostat but i was wondering why does the thermostat have two inpus when there is three cables?

You have the wrong stat.