Actually receiving and STARTING the checks is one thing. That's where the three every second comes from.
What the articles on the subject spun up (oh gosh, the media is anti-gun?) was the fact that a proportion of all checks come back with no immediate result and by law the FBI can only delay by 72 hours. That is the press/FBI definition of "unable to check". Reasons for this are legion, such as some states are flaky on submitting up to date records, their IT may be down when the NICS check is attempted, etc, etc.
Now, for a "no result" check, after 72 hours it is up to the dealer if they wish to proceed with the sale. The FBI say they do not know if the sale proceeds, so they take their "worst case" which is they all do, and they define them as "gun sales with no check". That's where the numbers come from. It's all BS and spin, IMHO.