I haven't worked on a Pro X3 myself, but try this link from Fuzion: http://fuzionscooter.com/pro/how-to-adjust-your-break/
How to Adjust Your Brake
Requires: 1 Phillips Screwdriver, 1 Small Adjustable C-Wrench or Needle Nose Pliers work as well (to hold the nut in place).
1) Remove the back wheel so you can access the brake screw nuts inside the deck.
2) Peal back the grip tape at the back of the deck until the screws under the grip tape are exposed. If you do this slowly you will not damage the grip tape and it will stick back to the deck once you are done.
3) Use a Phillips Screwdriver to tighten the screw. You might need to use the Small Adjustable C-Wrench/Needle Nose Pliers to hold the nut while you tighten.
4) Once tightened, your brake should no longer be loose. If your brake is still loose, you could add a washer to the nut side of the screw so that there is more surface area being tightened on the brake piece.
5) Put the grip tape back down over the screw on the deck and assemble your back wheel. You are good to go!