Im thinking the 3rd black wire is a dealer installed or connected option?
Similar thermostat
thread (though faulty Tstat) showed the 3rd unknown black wire on an ArticFox camper.
The basement fan sounds plausible. If it were mine Id find what is being energized by the tstat throughput. Mostly out of curiosity but to see what kind of load is being placed on the tstat contacts?
An even older
thread by deltabravo replacing same tstat with a programmable unit on an AF shows same crimped on 3rd wire connection
I dont know if the tsat he used is still available-
