Insigne and Berardi should be fighting for the starting spot in the front three, Chiesa should be a lock down starter either on the left or the right
Insigne is the best forward by far for Mancini. He is central to the way the team plays during the build up and is in the best form of all of them as well.
Its Berardi vs Chiesa for the right side, or both of them together with Chiesa on the left when Insigne needs to rest. Berardi is ahead of Chiesa but there is a also a matter of style, it should depend on the opponent and the moment of the match.
To put it into perspective
Insigne has 11 G+A in 11 games played since the euro qualifiers started. Only in 1 match he didn't score or assist.
Berardi has only recently joined the team, 5 G in 6 games. Again didn't score in only 1 of them.
Chiesa hasn't been as prolific, 4 G+A in 13 games, none in the last year and a half
Now Chiesa has upped his game so I expect him to come through big in the euros but he starts behind the other 2.