FREE NOW taxi app in Rome

I used it in Scotland and had no problems.

There are fixed rate taxis from FCO into Rome. Go to the “official” taxi queue. Don’t use anyone who approaches you about a cab. https://www.rome-airport.info/rome-airport-taxi.html

Not sure where you are finding fares, but it is not 28 EUR, the cost is 8 EUR so 24 EUR for the 3 of you, plus 1.50 EUR each for bus, tram, or train from Trastevere Station. Even the Leonardo Express is only 14 EUR.

But that said, depending where in the Gianicolo neighborhood, a taxi at 48 EUR might be handier, as some parts of the area has less access to public transport. If you are near the river or San Pietro Station, then public transport would be fine, but might involve more than one transfer after Trastevere station. Google Maps could probably lay it out.

If coming from the airport though, I would go to the official taxi rank and get the flat rate, I would be amazed if the fare wound up being less than the flat rate, very likely more. Edit: FreeNow is more payment app than hailing app, if you accept a cab outside of the rank, using FreeNow, they are under no obligation to stick to the flat rate.
Verify with the driver, given the address (flat rate does not apply to everywhere), and that they take credit card (They do, but then claim their machine is broke once you arrive, make sure they know you have no cash). Verify rates before you go, all of them seem to have gone up in the last year.

Sorry, my mistake. I had queried for 3 passenger while searching on Trenitalia website and forgot about it. Fare is 8€ indeed. Thanks for pointing that out.
Our route involves changing trains at Trastevere. I am thinking of just hiring a raxi from there to the Airbnb.

A method I often use is to take the Leonnardo Express from the airport to Roma Termini, and then take a taxi from there to my hotel. There are lots of them waiting outside the front door of the station. You can buy tickets at airport but be sure to validate before boarding!

Of course you could also use one of the official taxis from the airport - https://www.adr.it/web/aeroporti-di-roma-en/pax-fco-taxi?p_p_id=it_txt_adr_cookiepolicy_web_portlet_AdrCookiePolicyBannerPortlet&p_p_lifecycle=0&p_p_state=normal&p_p_mode=view .

We used it this last spring to arrange for 5 of us (with luggage) to get to the train station early in morning. The car arrived promptly and price charged was as quoted. Very good experience.