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The guy who played Three Dog in Fallout 3, tweeted (I believe that was the method he used)...that he was working with Bethesda on a "secret" project.
Now it is possible he is playing a different role in a new IP...but, this screams Fallout 4 to me. Especially with Bethesda's announcement of them about to reveal their next big project. It makes the most sense that it is Fallout 4, and having Three Dog come back.
Plus, "everyone" is wanting a Fallout 4 anyway. And I would love to see what they could do with their new engine.