Sure, but the problem with that argument is that if they are in fact off, they are almost always off to the high side, meaning if your speedo is indicating 70 you might actually be traveling 66 or so. I don't think I've ever seen a speedo that indicates slower than you are actually going. Therefore, if they tag you for going 3 over, your speedo was likely indicating 6 or 7 or more over, which means you should have reasonably known that you were speeding, no matter how small an amount you were actually speeding by.
Don't get me wrong, I think zero tolerance sucks. Pretty much across the board for years police departments have allowed up to 10 over the posted without writing a ticket. To now change that will catch a lot of people out who are not expecting such a change. But your speedo being off actually works against you as I have never heard of a speedo indicating you were under the posted when you were actually traveling over the posted.