2 or 3 prong adapters?

I agree with Ed. I have always just needed a 2 round prong adapter .... in Italy
I carry 2 and they are so small, they fit in my toiletries kit or in the toe of a shoe. Make sure you don't forget it and leave it in a wall plug in your hotel room.

What does each one cost, a couple of bucks? Make the rest of your life easier. Go down to Radio Shack or any place that sells electronics, travel junk, luggage, K-mart, etc, and buy a set of world-wide travel adapters. Also pick up a wall usb plug. Now you've got maybe six pieces that you can cobble together. You'll be set for the rest of your life no matter were you go.

The short answer is that there is no short answer. We stayed in one place in Rome that had outlets that required three different types of plugs. The excellent Ron in Rome sheds light on this subject (pun intended). He lived in Rome for years and has about the best blog I've found on Rome and Italy. Here's what he says about your question: Power in Italy

Nikki, The answer will depend to some extent on whether you're connecting devices which have a two pin Plug, or those with an extra grounding pin. It is possible to use the two-pin Euro Plug Adapters with grounded Plugs, but the grounding pin stays outside the Adapter so it isn't connected. I prefer to use a grounding-type Plug Adapter for those appliances that are designed with a ground, so always pack along both types. The grounding Plug Adapter for Italy is similar to the non-grounding type, but has three pins inline, with the centre pin being the ground. Happy travels!

If you go elsewhere on this web site to: "Travel Store > Accessories > Handy Extras" you will find travel electrical adaptors and they are currently on sale for $1.00 each.
Buy a few just in case you leave one in the wall socket.