Straight from bethesda's website:
https://bethesda.net/#en/events/game/fallout-4-add-ons-automatron-wasteland-workshop-far-harbor-and-more/2016/02/16/77
At the bottom of the article you can sign up for a free dlc beta. https://account.bethesda.net/en/settings/beta/fallout4dlc/signup
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They said the 3rd DLC will be it's largest new area DLC ever for them. Just think about how incredible the Skyrim DLCs were. You got to become a vampire overlord with a whole new island, ride dragons, new weapons and gear, a bunch of new story content and side missions. Bethesda has had some incredible DLC.