Arriving Terminal 1 from US and connecting to ITA flight in Terminal 3. Will I need to go through security? How much time should I allow to connect to the next flight?
You will go through immigration in Rome and probably customs depending on your connecting flight. And yes you will go through security. I would allow at least three hours. If less, you can enjoy a nice coffee or beer while waiting. You asked the same question, last month. The answers are the same.
It is a nice airport with the restaurants one level up above the main concourses.