How can you wire a 3 wire attic fan to a 2 wire thermostat?

If your thermostat is designed to switch the fan on and off at say 120 volts, then it acts just like a switch. This means that you insert the two wires from the thermostat in series with the black wire going to the fan. So let's say you have a supply of power for fan that has black, white and bare wire ground. You connect the white to white on fan and ground to ground. You then connect one wire of the thermostat switch to supply black and one to fan black wire. If you are unsure of the type of thermostat or anything else get an electrician. This is just one possible solution given the information provided.