Problem of the Fortnight #9

Evil Genie!

An evil genie wakes you from your slumber and taunts you with the following puzzle:

“I have 12 stones here that are identical in every single way except that one stone is either a bit heavier or a bit lighter than the other 11, I forget which. I will give you $1,000,000.23 if you can determine which one the unusual stone is. I will only let you use this balance pan scale, AND you can only make three weighings with it. Please tell me how you would use the three weighings to find the unusual stone. Mwuah-ha-ha!”

Solutions are due by 4pm on Friday, February 10^th in Goodrich 108