Important point about why Comey had to send this letter, and why today.
Comey testified under oath to Congress that the investigation was "complete". The letter is an amendment to that statement. And as that was under oath, he could theoretically be under some legal jeopardy if he did not issue timely notification that his testimony had to be modified.
That could tell us that there's less here than we think, but it seems this letter is purely for Comey to covering himself with Congress.
If it was only three emails as has been reported why couldn't he have quickly reviewed them for classified information and then reported the results of that with the letter to congress .......... instead he leaves it deliberately vague and open to politicking from the GOP.
Hope Obama or whoever tears him a new one.