Arriving in Nice and flying same day to Rome

Where are you arriving from?

The Nice airport is small, and you have three hours. So, normally, no problem. But you have variables that can eat into your time: plane's actural arrival time, weather, going through passport control or not, and going back through security. All said, you have an excellent shot at succeeding.

from the other threads, maybe OP is arriving by train or very short hop flight from Alsace or the Black Forest... in that case would also need to get to the airport.

I think we need a little more info

We're arriving at the Nice airport from the US. I guess 3 hours should be enough since it's a small airport. CDG might be a different story. Thanks everyone.

I should have added that we are doing Italy first, then spending the last 18 days in France

Nice is small and easy, including the immigration process if it's your first Schengen destination. I flew there from London and I was out on the street in less than half an hour, including retrieving a checked bag. Three hours should be plenty.

I don't know whether there are nonstops between Nice and US airports (I'd be surprised). If you have to connect somewhere before Nice, you'll want to allow enough time for that connection too.

Delta has a nonstop flight from JFK that arrives in Nice at 8:15 am and a nonstop flight from Atlanta that arrives at 12:00 noon.

The only 1245PM arrival I can find for a DEN-NCE routing is on Lufthansa via MUC.

That being the case immigration/passport control will be at MUC and the transfer at NCE will involve no border crossing formalities.

Just a matter of the MUC-NCE flight not being horribly late, OP should have plenty of time, to check the bags and get to the gate before 320PM. ( "Gates close promptly 30 minutes before your departure."--https://www.easyjet.com/en/help/boarding-and-flying/boarding-and-on-board-services)

If DEN-MUC-NCE-FCO is what OP is doing, I am left wondering why the why the detour through NCE. Presumably OP found some benefit to doing that, that is not apparent to me. As best I can see from a cursory look DEN-MUC-FCO and DEN-MUC-NCE are virtually the same cost.

When I was looking Den to Rome and Florence was more expensive than Denver to Nice. We really wanted to go Denver to Florence, and skip Rome. We are spending time in France also but doing it last as it will be warmer in Italy. When looking for a flight to from Nice to Florence, it was more than 3x as much than flying Nice to Rome. The flight to Florence goes through Rome 1st. So we decided to spend a few days in Rome and take a train to Florence from Rome. After booking everything, 2 days later the fares from Denver Nice, Denver Rome and probably Denver Florence all dropped about $300. Such is life. Now I'm trying to figure out how to get back to France, Train or plane. We really are on a budget. Do we want to spend so many hours and changes, possibly stops on the train, or hassel with getting to the airport and way ahead and Florence to Nice or Lyon or Paris isn't so cheap by plane.
Of course by train, which we enjoy, we could stop in some interesting places on the way, depending how many more days we want to take away from our time in France. It's fun but a lot of work planning a trip when you have to have lodging reservations everywhere. Perhaps in some of the small towns we could find an airbnb a day before, but do we want to risk it? Has anyone tried it?