The NICS info does not say they will IN FACT get back to the dealer in the three day time frame. I have heard of some lengthy delays before.
Some dealers have a policy that states they will not transfer a firearm unless they get the proceed from the NICS. In that case, you either wait or move on.
That being said- I recommend filling out a VAF (voluntary appeals file) once everything gets straightened out- find it on the NICS site linked to above. This will give you a UPIN for future purchases and hopefully eliminate the delay.