Italy 3 Days Pass

For trips involving Italy only I would not buy any railpass:

• If you use a train which does not require any reservation, the fare does not justify the railpass, it’s so cheap.
• For faster trains you will have to make a reservation anyway, do you have a railpass or not. The best is to make a reservation just before you take that train - you have to be flexible.

So you have to go to the ticket office anyway. You do not save any time and the potential savings on the fare rarely if ever justify the hassle to get a ralpass and to get the mandatory reservation.

Longer trips in Italy or numerous trips involving multiple countries within extensive period of time is a different story. For your trip do not worry about the train tickets, just buy them at the station when you are there. The trip from FCO to Venice is a good example, why you should not over-plan: you can’t be sure, your airplane arrives on-time, you can’t be sure the luggage is processed on-time, you can’t be sure, there is nothing unexpected regarding COVID at the airport. You could be delayed - or if you arrive from overseas you could be even early. You will know everything only when you are past the customs and collected all your luggage. That could be 12:30 PM or could be 2:30 PM. It would be a mistake to even try to reserve anything ahead and you could be even better off to take other trains or use train transfer.

One more thing: Venice is a tourist trap, you are probably aware of it. Your train probably will pass Orvieto, which is a wonderful city with ancient cathedral. If you are interested in early Christian art, the city of Ravenna is far more interesting, than Venice (just an example). The trains reaching Ravenna are all non-reserved, local cheap trains… (Just giving you an idea).

On the other side in these COVID times visiting Venice could be an excellent idea: much less crowds.
But just FYI: the mosaics at St. Marc are a far, far cry from what you could see in Ravenna,