1) 3DC when your oppo has not offensive wings (that includes M) and at least 2 S. 3W otherwise but no more than 1 S. If S >1 go for 4D.
2) DMC if he's got AM, even wingers
3) Not sure it's the most widely used but it's one formation with no obvious weakness. Like 4-4-2 for instance. But to me, 4-5-1V is vastly superior.