As you travel just in Italy and wanna use Highspeed trains you may use the Trenitaliapass (Instead of Eurail Italypass) The trenitaliapass works per ride but includes reservations while the Eurailpass works like a dayticket and you need Reservations on top of the Eurailpass (13€ per person/seat)
If you have fixed dates and doesnt want any flexibility you may even end cheaper with Advancefares of the Italian Railcompanies (On the mainlines you have 2 operators Trenitalia (State Owned and usefull when you travel on regional routes but they even offer a superb Highspeed experience) or NTV Italo (the second Highspeed operator in Italy on mainlines like your mention routes)
What is finally the best depends on your preference (save money, flexibility)
In general Advancefares are the cheapeast option but often not refundable or exchangeable
A railpass like the Eurailpass = a mix of flexibility and saveing money
The Trenitaliapass (is even more flexible than the Eurailpass).
I recommend to watch few tripreviews of the “FrecciaRossa” Trenitalias Highspeed service and Italo the private highspeed service to find out which trains you finally wanna catch.
High Speed trains in an Italy are great. On time, comfortable and very frequent.
As @seewulf says, there are a few options that might suit you. A Eurail Pass, a Trenitalia pass, or just buying individual tickets - especially if you are happy to lock your travel in early for a good price.
What dates will you be travelling? You can look up individual train times and prices on www.italiarail.com - all in English.
Those are all very easy trips, with fast good quality trains.
As an example, Milan to Venice is about 2.5 hours. You could buy a ticket for tomorrow for about €55, or in a months time for €17.50. That’s likely to be better value than a rail pass, if you don’t mind booking in advance. And it still comes with some flexibility to change plans.
I’ve done a quick check.
The cheapest Eurail Italy pass would be ‘5 days in 1 month’, costing €228, plus €13 reservation fee for each fast train.
If you bought a Trenitalia railpass, you’d be looking at €246 (with no reservation fees) for a ‘7 trip in 15 days’ pass.
In both cases, I think you’d be much better off just buying tickets instead. It’s also simpler!