That third prong is an equipment or safety ground. It's purpose is to provide electricity an alternate path to ground besides thru you in the event that you are handling some appliance that suffers a fault.
You should invest in an inexpensive non contact voltage tester and identify the hot wire with it and install it on the correct terminals of your outlet.
By code, you can replace a 2 prong ungrounded receptacle with a gfci only if you clearly label it and any others connected to it's load side as "no equipment ground present"
The larger issue is that knob and tube wiring. It was never designed to carry the loads that a house full of appliances put on a circuit and, particularly if it's been interfered with or added onto, it is a significant fire hazard.