I actually thought that the 3rd secret was known to the church.
It was revealed several years ago.
The conspiracy-minded say that (1) the Church never discovered the third secret and Sr. Lucia took it to her grave because the evil men who run the Church never let her tell it or (2) that the Church knows it, but the evil men who run the Church have kept it hidden and that in either case, the divulging of the secret a few years ago by the Vatican was a fabrication.
Those who are swelled by their spiritual pride into believing their own fantasies will never be satisfied.
I expect some Dan Brownesque charlatan to make some quick bucks off this situation in the near future.
Handwriting Analysis of the Alleged "Third Secret":
http://www.tldm.org/news/lucys_writing.htm
It's from a Leukenite website, but in this instance, I think they're correct.