So far it seems the Scudetto competition in this season will be for three clubs, AC Milan, Inter Milan and Juventus. Other clubs like Napoli, Fiorentina, Atalanta and Roma will compete for one or two positions in top four and top six of Serie A.
I think it is too early to judge only 3 teams joining the race of the champion. It is still matchday 9, we are very far from the end of the competition. At least, we should be in the second half of the season to know which teams to be the strong candidates for the champion. So, I think al the current top 5 still have the same chance to be the champion.
The appearance of Fiorentina in top of Serie A is surprising even they got a Conference League final match in the last season. Because they lost Amrabat, one of their most important players to Manchester United in the summer. Anyway it is a good continuation of Fiorentina with good improvements during last three seasons. Even transfers of Vlahovic, Amrabat don't make them weaker and their performances in this season are very good quality mark for their team.
Fiorentina has a good management to replace the key players who leave the team. Although there is no popular players in Fiorentina, but they still can play solidly. I think they can maintain their good performance because they always play as a unified team and they plan with a high confident. The coach also apply the wright tactics for Juventus squad.
Participation of competitors like AC Milan, Inter Milan and Napoli in Champions League will give Juventus more chance to take Scudetto in this season. Champions League is like a magnet for all clubs and competitors of Juventus will be distracted from Serie A in weeks with Champions League matches.
I don't think joining UCL makes Inter, Milan, or Napoli to lost their focus. It is not the first time that they play in UCL while playing in Serie A, too. They have enough players to play in several competitions. So, we don't need to feel doubt about their focus, they are familiar with the situation. They know what the solution to apply.