Two or three prong adaptor for Italian sockets?

You need the three pronged adapter.

I have always used the 2 prong europe adapters in Italy for phone and tablet with no problem. Grounding plug not needed for small electronics.

http://store.ricksteves.com/shop/p/european-power-adapter

I have always used the two prong adaptor for Italy and take several along.

Only took two prong to Italy in April. Worked fine everywhere we were.

The third (middle) pin on an Italian adapter is the ground. If you look at the nameplate on the USB charger you will probably see a square-in-a-square symbol. That means it is "double insulated" and does not need a ground.

Your phone and tablet chargers should probably only have two equal width prongs (blades), particularly the one for the phone. What kind of plug does the charger for your tablet have? If your tablet charger does have three prongs (two blades and a round pin), then you should use a three pin Italian adpater.

The two pin Europlug adapter is ungrounded and limited by code to 2½ amps at 230V (less than 600W), which should be fine for charging.

Two.
Unless your appliance has the ground pin.
In that case use the 3 prong adapter.
Tablets and phones have two.
Some computer chargers have 3.

For your needs, only two.

Last trip I bought a 2-USB port charger made for Italy. I only take that. Eliminates the US plugs.

2 prong. We are in Sicily at the moment and everywhere we’ve stayed 2 prong works.

Hmmm. Learn something every day. Not being an electrician, my answer was based on the Airbnb’s I’ve been in have 3 prong adapters on their European-made 2 prong lamps and hairdryers. I always just went with that as proof.