Dropping rental car off at Naples airport before catching flight; is three hours enough time?

The airport is small and quick. Car rental is big unknown. Personally, I would drive to Naples the day before your flight and return car and stay in Naples. Then take a taxi to airport in the morning.
I just didn’t find Naples worked as smoothly as you might hope. It is part of the charm when you have the ability to be flexible but you would not.

Good advice. However, driving on the Amalfi Coast is not recommended. Read this as one example:

https://www.tripadvisor.com/ShowTopic-g187779-i161-k11500341-Driving_the_Amalfi_Coast_advice-Amalfi_Coast_Province_of_Salerno_Campania.html#91428089

I was going to say that too. We took buses, trains, and ferries. At the hotel we stayed in Amalfi Town I still remember the pale faces of the couple who had just driven to the town.

It has never taken me 2 hours to return the car anywhere including southern Italy. If you give yourself 30 min to return the car, it will be more than sufficient. Check in should be done at least one hour prior to departure. So if you return the car before 8:00 you will have plenty of time to spare.

We're big proponents of minimizing the drama on getaway day.
Turning the car in the afternoon or evening before your flight and staying in Naples would certainly streamline the whole process for you and minimize the chance that an unforseen delay would spoil your day, if not your trip.

easyjet closes the door 30 minutes prior to scheduled departure. You need to be ON the plane at 10.

If anything happens on the way to delay your arrival at NAP, life gets extremely 'interesting'.

If going to Naples the night before is not an option, then going to Rome for a later flight makes sense only if you choose the 5PM flight.

For more on driving on the AC, read my post here. Note that I was there in February when there are few tourists and hardly any traffic on the road. While May is probably not as crowded as summer, there will be much more traffic and much less parking than in Feb.

My experience with returning cars at European airports is that it is fast and easy. Pick-ups can take a long time, but not drop-offs. OTOH I've never used Sixt.

I remember what chaos there was at the car rental return in Naples when we drove there from Umbria. We were staying in Naples so had no issues about catching a flight.
Later in the trip when we flew out of Naples, the airport was easy and efficient, no problems.

I wouldn't trust Google Maps drive times. I doubt if 1 1/2 hours is enough. On a Saturday morning we drove from Naples Airport to Mongrazzano Calabria and back on Sunday afternoon. If you're driving during a weekday, I'm sure you'll need more time. We had no problems driving down using WAZE. Going back to the airport to drop off the car, we got very lost and ended up down by the docks because we stayed on A3 the west side of Vesuvius and couldn't find the airport exit. We missed the exits to take us to A30 at Salerno which would have been an easier route to the airport. Give yourself plenty of time to find a gas station to fill up the car and to get lost. When we got near the airport the only gas station was on the wrong side of the road. Luckily we had filled up just before Salerno. The Hertz check-in people took pity on us and let it pass, probably because I told them I was not driving anywhere more after getting so lost and they could charge me the €130 fill-up fee if they wanted (old lady frustration tears at this point). Car return took probably about 20 minutes. Then we caught a cab to the hotel next to the train station for our trip to Venice the next day.
I hope you're not driving on the Amalfi Coast. The road is narrow, curvy and slow. Being on a tour bus and then the local bus was bad enough. Take the train from Sorrento to Naples as others have suggested.

Just take public transport and ditch the rental vehicle. You’ll save yourself a multitude of headaches.