Hey Steve, i have had great success with just the 3 percent. Let it set for at least 24 hours. If you can't get the entire skull submerged use cotton balls or a paper towel to lay on the portion that is out of the peroxide. The cotton balls or paper towel will wick the peroxide onto that portion of the skull. you can also rewet the area every couple of hours if need be. Afterward, set the skull on a white sheet or pillow case in the sun. The sun will help the process of whitening.
Good luck and let us see the final result.