The final Halo 3 ViDOC is up on Xbox Live Marketplace "Journey's End."
Update: Read more to see ViDOC!

"Journey's End" focuses on the story leading up to the events that unfold in Halo 3 -- there aren't any major Halo 3 spoilers in the ViDOC. However, if you haven't played the Campaign yet or are expecting it as a present for the holiday season, our Marty-esque advice is to unplug from the Internet and not watch the ViDOC until you've played the game.
via Bungie.net

Part one:


Part two: