I don't know Corsica, so can't comment about it. Over the years I have been a tourist for a few days in Florence twice, and in Rome and Milan three times. All were enjoyable.
Florence and Rome have more visual impact than Milan, unless you are interested in fashion.
If these cities are new to you, it is good to remember that the famous sights are famous for a reason. See them, by all means!
For a few suggestions that are not at the top of the list in guide books, I found Trastevere an interesting section of Rome.
Santa Maria in Trastevere by Danny Watson, on Flickr
The Renaissance architecture is the main draw in Florence, but there are also some opportunities for photos from more open spaces.
Florence green space from Boboli by Danny Watson, on Flickr
Della grazie bridge by Danny Watson, on Flickr
[These shots from 10 years ago were with a point-and shoot.]
Of course Andrea is a regular forum contributor who definitely knows Florence
You can't go wrong with any of the cities you mentioned. Have a great trip!