I have a flight from Athens on Aegean to FCO and then a flight from FCO to the US on United Airlines. I believe I will need to go through customs/ immigration and collect bags, then re-check in at United and go through security again. We have a 3 hour layover in Rome. Is that enough time?
Are both flights on the same ticket or are you self connecting?
2 different tickets so self connecting.
3 hours is enough as long the first flight arrives on time.
Then it should work, but there is always the risk that your first flight is delayed.
My experience last month flying from FCO is that three hours is enough time even with two tickets. Caveats include the first flight arriving on time and the baggage being delivered promptly. Boarding may start almost an hour before departure. You (and baggage) must be checked in a minimum of 75 minutes before departure.
https://www.united.com/ual/en/us/fly/travel/airport/process.html