Italy being a fairly major european country where football is the no 1 sport I would think the infastructure is already in place to host big tournaments.
Stadia wise
http://en.wikipedia.org/wiki/List_of_football_stadiums_in_Italy
They have quite a few 40k + size stadiums but how many of them meet all fifa's other criteria I don't know. It could be a case of only a few having to be modified and, if so, they wouldn't have to worry about building any stadia. In that case 3 years could be plenty of time.
Of course there are a lot of if's there but having the 80k San Siro already would be a big asset in the event that they decided to bid.