I'm curious, do you have the main page for the 3rd gen as the link in your "Favorites"/bookmarks?
If so try going to the main forums listing page and then click on the 3rd gen section and see if you're still having the same issues, if not update the link in your bookmarks/favorites.
I was having some issues after the latest server upgrade with the main forums page showing as not being secure using my favorite/bookmarks link and I was logged out everytime I visited the forum, e.g. the "Keep me Logged In" box didn't seem to be wroking, I then clicked on the 4th gen section and it was secure, I then when back to the main page via the link at the top of the 4th gen section, the main forums page was then showing up as secure, I noticed that the link in my favorites was just a little bit different from the secure link after going back to it from the 4th gen section, so I updated my favorites link and from there on I haven't had any of the same issues since updating the link in my favorites.