What's the formula for cutting vinegar acidity? I have a vinegar based bar-b-que sauce recipe that calls for 3% acidity vinegar which cannot be found. How do I cut 5% vinegar to 3%?
Cut it with water.  

Commercial white vinegar is basically nothing but water and acetic acid.

3 units of 5% vinegar and 2 units of plain water give 5 units of 3% vinegar.