3% is plenty strong enough! Don't waste your money on expensive, dangerous peroxide when the cheap Walmart stuff does the exact same thing. Wrap the skull in peroxide soaked rags, then cover with plastic to prevent from drying out. Give it about a week. However, because the skull has been in the dirt for so long it will be stained and it isn't ready for whitening yet. Try a long soak in water/soap and see if that pulls some dirt out.