Both Juventus and Inter Milan are putting in a lot of effort to ensure that they continue to play well. Both teams are capable of winning the league title, but Inter Milan is the clear favourite because no team has defeated them thus far this season. Juventus is also playing well and is ahead of Inter Milan by a few points, but I still don't think they can win the league title.
Regarding Napoli, I don't think they can win any trophies this season because it is very certain that they will be unable to do so. Last season, Napoli was the favourite team in the serie A league, but this season, they really let people down with their play; they are the team that didn't sell out many players and even their best players don't look good. Napoli is currently in ninth place in the league table, so it is not even safe to say that they will be in the Europa League.
Juventus is working very hard and in a convincing way, we can`t say the same for Milan, I see them limping and not convincing either in their play or in their results. Napoli, well, for me they don`t even reach the cups this year, i mean cup`s zone (cause points).