There's no right or wrong answer, except that saying never or always swing on 3-0 are both wrong. It depends entirely on the situation and the players involved.
Never swinging on 3-0 is like never swinging on 0-0. While you shouldn't do it frequently, if you never do it, you're handing the pitcher an outsized advantage. You have to do it at least once in awhile just to keep him honest.