With just a few supplies and about 10 minutes, you can add wireless charging to your S3 for an easy $25. The hack is thanks to a member of the XDA Developers forums and Palm's wireless charging kit for the now-retired Palm Pixi.
Follow this slideshow for the step-by-step instructions.
Start by peeling the black lining off the Palm Pixi case. You should be able to do this easily with just your fingers. Then, remove the silver adhesive. Save both of these items -- you'll use them later.
Grab a small knife and carefully remove the circuit board and the copper coil. These are the parts that allow your phone's battery to receive a wireless charge. Finally, remove the metal discs and set them aside.
Then, before you remove anything else, grab a small piece of paper (translucent wax paper works here), lay it on top of the circuitry, and carefully plot where each component is. Be sure to include the coil and the four metal discs. Set the template aside -- you'll refer to it again in a bit.
Remove the backplate from your S3. Place the coil upside down onto the inside of the backplate, with the copper coil about 1/4" from the bottom edge of the cover. Then, bend the L-shaped circuit board up. This puts it in a position that allows it to make contact with the phone.
Next, replace the metallic adhesive over the coil, using tape if it doesn't adhere. Then, using the template you made in the first step, place the metal discs around the copper coil. These will keep your phone attached to the touchstone. Once everything is positioned, tape it down with scotch tape.
The final step is to bridge these parts in the back cover with your S3's battery. For that, you'll need to grab the copper tape. To make this process easier, prepare four 1/8"Wx1"L strips of copper tape.
On the L-shaped part of the circuit board, you'll see two raised power terminals. Grab a strip of copper tape, and fold over about 1/8" of one end. Place the folded-over end on the top power terminal, sticking the rest of the tap down so that it reaches the edge of the backplace. It should basically be parallel with the bottom edge of the backplate.
Then, take another strip of copper tape and fold it over the same way, placing the folded-over edge on top of the bottom power terminal, but this time, stick the tape down diagonally toward the edge of the backplate. Make sure both folded-over ends are pressed well against the power terminals, even if they don't necessarily adhere.
Finally, grab your phone. To the left of the battery, you'll see two power terminals. Take the last two pieces of copper tape, and, repeating the method from the last step, fold over one end. Place the folded-over end in to the top terminal, and repeat the same thing with the bottom terminal.
With a pencil, push the folded-over ends copper tape into the terminals just a bit, so there's a good connection with the circuitry.
Reassemble your S3, and place it on the Touchstone -- you should get a notification letting you know your device is charging. Congrats!
With your phone charging wirelessly on the Touchstone, it will charge at about the same rate as if it was plugged in via USB.