From what I understand, the 3-day wait should be preempted by the state, since that's regulating possession, which is specifically listed as off-limits to counties. I don't know if anyone else has really thought about that. If the county says that it's something to do with registering the first handgun, they're full of crap. The dealer submits the registration paperwork after you take possession anyways. I didn't even fill out the registration papers until I came to pick it up.
Unfortunately, to challenge this by action, we'd have to find a dealer that is willing to hand over the gun without waiting 3 days. I don't think many will help, since they don't want to lose their business licenses.