Three Dog was one of the more memorable characters from Fallout 3. If you somehow forget Three Dog, he's the DJ for the Galaxy News Radio station in the Capital Wasteland. The Capital Wasteland is set around the ruins of Washington D.C.
Three Dog himself isn't in Fallout 4 (at least not yet) but he is mentioned by Travis, Fallout 4's resident DJ, after completing a certain quest.
Three Dog is still one of my favorite characters in the Fallout series and I'd love for him to make an appearance in Fallout 4 and its DLC. It could just be one massive quest, it could be an entire DLC expansion centered around Three Dog. I'm joking about that last part. Or am I?