I was an IBEW Electrician for over 20 years before I was injured, and a 3 way switch is actually used to turn a light fixture on from one of two locations, like at each end of a hallway. For the application you are wanting to do you can either add another on/off switch to the existing switch giving you one for the lights & one for the fan, or you could possible find a 3 position switch that one position would be off, next would turn the lights on and the 3rd would turn on the lights and fan. In your example of what you are wanting to do, flipping the switch to the 3rd position would turn the fan and lights on, and after you are done painting flipping the switch back to the 2nd position would turn the fan off but leave the lights on. I'm guessing that the paint booth you have uses a rocker type switch currently, and you can probably find a 3 position rocker switch close to the same size to fit into the existing opening. However, either way you decide to go, you will need to open the unit up and trace out the wires to find how they go from the switch to the lights and fan and see if it is possible to wire each separately. If you are not experienced with electrical work I would find someone more experienced to do the work, especially if there is any sort of printed circuit board inside. If the unit is still under a factory warranty, modifying it will void it. If you decide to try the 3 position switch, there are a few electronic supplier websites that you should be able to find it at, just make sure to get one rated for the voltage and amperage(current) needed.