I think each of them would chip in with 10-15. Diego can easily reach double figures. I think having those two up top you would leave alot of space for the likes of Diego to exploit it would be a deadly combination for sure.
In order for the middle of the pitch to open up for Diego, wouldn't you have to have forwards who have the ability to do so, with pace and movement?
If that is true, then you are counting on a player who was never known for his pace (Cassano), and a 36 year old striker (Del Piero) to provide the separation.
The all seem to work their best in confined spaces, suddenly turning something out of nothing, which is fine if you don't have other players with similar qualities.
I can just envision a 3 man traffic jam just outside the box with these three. I don't think it will work.