My point was, absent an announcement from Groundspeak, I do not believe it is possible to determine which cache was actually the 3,000,000th. If they were just counting the number of geocaches published, I'm sure someone could work it out. But between the archival of existing caches and the publication of new caches, #3,000,000 was a moving target, extremely difficult if not impossible to determine by anyone outside of Groundspeak.
I would assume if Groundspeak wanted to announce it, they would have done so at the time they celebrated the milestone. Since they didn't, I further presume they won't. Perhaps it was difficult for them to determine as well, or perhaps it was a low-quality cache they didn't really want to advertise.
tl; dr: ¯\_(ツ)_/¯