General rule seems to be 2 inches in either direction. I'm inclined to say 3 inches is too much; when you're taking in the pants from just the rear, you're still altering the entire garment, e.g. the rear pockets are going to be brought closer together and the side seams are going to move rearward. It's really a matter of how the pants are designed, and whether the amount of adjustment will throw off other elements and how much.
I hope that makes sense...
EDIT: Noting PMRuby's post (which came in whilst I was writing mine), I was going to mention more advanced tailoring techniques, but often that can become economically inefficient, depending on your budget. So yes, it can be done, just depends on what you want to spend. And, by all means, follow PM's advice about finding a quality tailor.