Napoli play the 3 man defence very well. It's actually becoming quite beautiful to watch. When one of the CBs goes forward with the ball one of the wing backs just plugs the gap. When they're defending they can easily have 5 at the back with two sitting midfielders infront, but on the break they can turn it round really quickly and have options to flood forward with. Maggio in particular is fantastic carrier of the ball and the front 3 are deadly.
It'll be interesting to see how they get on with it outside of Serie A, they completely stifled Man City for the most part.