Where to go for three nights - Rome too hot

How Can you know today the temperature in Rome in 3 weeks?

If I got it right your idea is to take a northbound train from Rome's airport to the Lake Como near the Swiss Border.

Then you are going back south to Lucca (or Siena?) by train, to rent a car and drive around Tuscany.

While in Tuscany you'll be driving twice under the mountains: north from wherever you'll be Tuscany to Bologna, then south again to drop the car in Florence and fly to Switzerland?

It seems a little complicate, perhaps I misunderstood. Anyway I'd google "Lake Bracciano" before deciding to bounce up and down the Boot.

The first time we visited Rome, late September, there was an unbearable heat wave. We spent a day at Lake Albano where the Pope’s summer residence is located. It was such a welcome relief.

Well, I've found that Tuscany and Campania are very warm as well - even in May and September. Our solution is to select higher-cost or less-homey hotels that have modern Americanski air conditioning to escape to. We're not "pool people" at home, but in Italy, we dunk in the pool (if there is one) after a hot day of touring.

Alas, there is no assurance that a shore town will have a cool breeze on your visit days, although the probability is certainly higher than in a city. But how can you reject the many sights of Rome? Some museums are air-conditioned. I have also found that few places can match the beaches of the United States or the Carribbean. I found the pebble beaches of Positano unattractive. The views from Capri are great, and there are breezes up on the cliffs. But there are few beaches. And I don't go to Europe to sit on the beach or walk in a forest.

I haven't been to the Italian lake district, but I know it books heavily in the summer, and has many seasonal-only gardens and ferries.

Our standard work-around when we've been in places during a local heat wave has been to first book a place that has A/C, then get out very early - to watch the city waking up and then being first in line at whichever attraction we were intent on visiting that particular day. Then, after a light lunch, head back to our accommodation for an afternoon siesta before heading out again in the early evening once the worst of the heat has subsided.
The strategy worked well for us a couple of years ago in Florence when afternoon highs approached the temperature of the surface of the sun. It also had the beneficial side effect of avoiding the worst of the crowds around midday.

Orvietto, not far from Rome, is supposedly where Popes went to escape the Rome summer heat. In addition to the old town fairly elevated, there are some heat escape tour sights such as the tour of the caves that run under the old city and a famous well you can climb down.

Great advice from Robert. When we lived in Rome, that is how we functioned. The heat is one reason Romans eat so late, especially in summer. We never ate before 9pm and by then the city is lovely, cooled off by the ponentino (wind from the west), and eating outdoors is a treat.

Also, use taxis, not public transportation (you certainly do NOT want to drive in Rome!), especially during hot periods. Seek out the museums with AC for morning or early evening visits. Look for night time special events like those in the Forum. Info here http://www.viaggioneifori.it/en/. There are many companies that do walking tours after dark, too. Worth pursuing,

So your early, rest in the afternoon, go out at night. Works anywhere in Italy when it is hot.

Right now it’s hot in Italy, but there is no guarantee the heat will persist 3 weeks from now.
Your plan seems good as you planned it: Rome, then Tuscany by car, then fly to Geneva. The only issue I see is whether the Florence to Geneva flight (Swiss) will have resumed. That flight is not currently scheduled and has not operated since the start of the pandemic, so you may want to check.

If there is indeed no flight, taking the train from Florence to Geneva (or anywhere else in Switzerland) is reasonably fast (6-7 hours) and convenient, with just one connection in Milan in the best case, and another one in the small Swiss station of Visp otherwise. If your final destination in Switzerland is not Geneva, it will even save time.

We plan to spend a couple of nights in the Monte Argentario area before finishing our trip in Rome. I haven’t been there before but the area looks gorgeous, on the coast. Looks to be about 2 1/2 hours from Rome by car. Orvieto, as someone else suggested, is a lovely town and easy to get to by car or train.

Since you’ve been to Como, you would live Lago Maggiore, and way less expensive than Lake Como. Stresa is about an hour train ride from Milan. 3-4 nights would be a great visit. You don’t need a car as you can explore much of the lake by ferry. And the lake temperature early September is warm, and swimming in a crystal clear lake is Devine. We lived in Verbania for 2 1/2 years and always stayed put August and first half of September because the weather was gorgeous..

If it's too hot in Rome, it's going to be too hot in Tuscany (it's hotter in Florence than Rome right now). I would wager than by September, things should be normal.
But since you have been to Rome before and may just want something different, what would be appealing to you if it is still rather warm--seaside? It's only a few days, so I would not go as far as the northern lakes.

Thanks for the comments. Right after I posted my message I realized how dumb it sounded. 🤣. Will stay put in Rome for three nights.

Thanks for the suggestion on checking our Swiss Air flight. It’s already been changed twice. Started out as non stop, now have to change in Zurich. This flight is from a cancelled flight in 2020, so we are using a voucher.

Check out Umbria, Orvieto, Perugia, Assisi and more.

If by chance it is very hot when you are in Rome, you can visit the Colosseum at night. I loved the night tour, it was much less crowded, you get to visit the dungeons, visit the gladiator floor and is was much cooler.