As you can see I have a Y G W R/2 C/3 D/1 terminals on my HVAC. I am planning to change my thermostat to one that requires a c line. There are 2 spare lines available, however I am not sure which is the C terminal. This is a forced air heating (gas) cooling(electric) system. I am guessing that its the C/3 terminal. Any help would be appreciated.
The C/3 terminal on your control board is indeed the C terminal you should hook your spare (blue) wire up to. How do I know that? Well, your compressor contactor's coil needs to be hooked between Y and C, like a lightbulb connected from switched-hot to neutral with the thermostat from R to Y taking the place of the lightswitch going from always-hot to switched-hot, and we can identify the wire running off to the compressor from the Y terminal (its the other wire connected to the Y terminal, given that the thermostat is wired to it as well), so we know that the other wire coming from that compressor cable must land on the C terminal.
