Assumptions can get us into trouble. It could be that it is being used as a single pot with the other pins used for mechanical support and do nothing. It is also possible the wiper(s) interact with each other. What is the taper of the pot, is it log, audio, linear etc. A schematic would help resolve your question in short order. Will the Arduino add digital noise to the analog section, how will you shield the pot lines, both digital and analog.