A quick update for fellow travelers to Rome... Delta Air Lines now operates out of Terminal 1, NOT Terminal 3 for arrivals or Terminal 5 for departures. Today was the first day that Delta went live using Terminal 1, which was a rather confusing experience when I arrived to Fiumicino airport this morning. Travel sites will likely update in the coming weeks, however I wanted to pass this along if you are traveling on Delta in the near future.
Here's a link that offers more info: https://pro.delta.com/content/agency/us/en/news/news-archive/2017/february-2017/delta-moves-terminal-at-romes-fiumicino-airport-to-co-locate-wit.html