WC availability in Venice, Florence, Rome?

No toilets are not as easy to find in those cities. We went to the toilets on trains and at the train stations (we paid coins in Siena Train Station, that is all I remember) or when we decided to eat at cafes or restaurants.

And museums.
We always carry a handful of €0.50 coins -- "pee money"

Toilets are not easy to find. We availed ourselves of the facilities when we had a coffee or lunch or headed back to our hotel when we were nearby. Note that some cafes don’t have toilets (that would be illegal in the UK). We googled which places had toilets as I recall and there was quite a lot of information online.

The first rule of travel is never pass up a bathroom. Don't leave a hotel, a museum, a restaurant or any other facility without taking a look at their bathroom. I've never had a 4- or 5-star hotel turn me away. In Florence you can go into the Palazzo Vecchio, walk past the ticket sales and the lockers and use the bathrooms for free.

The good thing is that most all bars (coffee selling establishments) have bathrooms for customers and you can't go half a block without running across a bar. Go in and spend ~1 euro on a coffee (or more for a pastry or bottle of water) standing at the bar and you are a customer, you are in and out fast (as opposed to sitting at a table waiting for service), and you get a coffee out of the deal!

Unfortunately, I have had to become an expert on this topic.

My system is to be sure not to "wait until the last minute," then go into a bar --- my husband orders a coffee for himself and something like fresh-squeezed orange juice for me while I head immediately to the bathroom.

Always be sure to have TP and handwipes with you and handy. If you're carrying or wearing a lot of stuff, it's probably best to leave all that with the person you are with.

There is no way to tell by looking at or entering a bar what kind of bathroom it will have, how clean it will be, or whether it will be supplied with essentials. I have been surprised both ways many times, including by a hole in the floor inside a very modern-looking and clean bar. Just consider it all part of the adventure. Once, in a restaurant I asked for the toilet and was pointed to a clear glass booth in the middle of the restaurant ----- whaaaaaaat???????? ----- but it turned out to be an elevator to the basement where the bathroom was.

A bar that's away from a train station or a heavily touristed area is understandably more likely to be nice about letting people use their bathroom and less likely to have a line of people waiting. Likewise, any bar in the off season. Be sure to greet the person working in the bar, to be polite about asking for the location of the toilet, and to thank him or her.

Heed all of the above replies about always using the bathrooms in museums, etc.

All of the above, and don't forget about your friendly McDonalds.

In addition to all the excellent advice above, there are some department stores you may happen upon that have reliable facilities. Look for COIN in all three cities and La Rinascinte in Rome. Also the Galleria Alberto Sordi on Via del Corso in Rome has free, clean, toilets.

You are a woman with a similar bladder as mine. I make it a habit in trip planning to investigate where WC's are! We were in Venice in late April, and this is the info I got online: Water closets that cost 1,50 Euros are at:
Piazzale Roma
San Bartolomo
San Marco- Calle Ascension (go through the archway from the square, turn right, and WC is down a short alley on the left. I used this one.)
San Marco - Giardini ex Reall (on the waterfront next to the tourist office)
Bragora
San Leonardo
Rialto
Academia- on the Dorsuduro side of the Grand Canal near the vaporetto stop.
Additionally, at Santa Lucia train station where the price is .80 Euros.
We went to Ca Rezzonico, and there are clean, free toilets next to the gift shop and cloakroom on the ground floor. I used this as well.
In the Doges Palace there are attended toilets with a fee of 1.50 Euros.
In some of the places I mentioned, I had no specific location for the WC, but you could inquire.
As was mentioned above, always use the facility in any cafe or restaurant or bar you attend! You will be fine. Enjoy Venice!

For your second question, while almost all of the bathrooms referred to above (stores, bars, restaurants, museums) will be free, it's always a good idea to be prepared with €1 and €0.50 coins for bathrooms - just in case.

I didnt think that its much different than in any US city. There are few if any "public" toilets. You have to use them in businesses, stores, restaurants, etc., that are open and willing to let you use them.

There are some public restrooms (often requiring 1€ to use), but the easiest way is to enter one of the many coffee bars, or similar establishments, order something (mineral water, espresso, pastry, gelato, or whatever) and use their bathroom. Large department stores have free bathrooms too. Take advantage of bathrooms in museums, they are the cleanest generally.

I have not yet used this app from Charmin but plan to give it a try next month:
Sit or Squat: Restrooms Near Me!

Thanks to all our fellow travelers for the excellent suggestions so far!

Years ago on our first trip to Italy, my husband and I walked everywhere! At one point we were far away from bars, restaurants and museums and I had to go! There was a park and hubby and I were walking through when I told him (in English) that I needed to find a toilette. A homeless man who was laying on a bench sat up and signaled to the interior of the park. He kept speaking italian but finally in English said "come". We both followed him and lo and behold there is a building that was marked toilette. I went in and was surprised how clean it was.

When we walked back out of the park he was back on his bench and I gave him a few Euros. At first he didn't want to take it,
but I kept saying "Thank You" and "Grazie" and he stood up and shook my hand and took the Euros.

Funny how a 10 minute encounter sticks in my mind after all these years.

But now I pay more attention and use the facilities whenever I can! I might not be so lucky next time.

I have spasms of my bladder and it is awful to have to find a WC in a hurry, here are some more places I've found. There is a public library in Firenze by the brass pig, going towards the Piazza Signoria. There are some beautiful libraries housing great works of art throughout Italy that I've literally run into and of course the standard places that have plenty of restrooms...museums and restaurants. None of the churches have facilities so this is something to keep in mind when touring them or climbing the towers of the churches. The WC are usually a euro or two, but always clean and well maintained.

and there are always depends . . .