I'm a little bummed, but I can live with it. He was never exactly a major part of my gameplay and I don't imagine he will be. A lot of the times I'd be muting the game (except the sound effects) and listening to my own music anyway.
As others have said, 3 dog was great and I loved the character but him not coming back for the new installment won't be something that would turn me away from the game at all, we have to let go of video game characters after all.
I actually really enjoy Diamond City Radio even without 3 Dog so far, so no, I don`t really care that he is not in this game.
Really? I have met him, but I did not get any quest. He seems very very shy, though, for some reason. Maybe I`ll get the quest from somebody else.