Venice and Florence

Both Venice and Florence have rather compact city centers that are good places to stay. I would suggest going to Booking.com to look for rooms. I double check places I like on Trip Advisor to see what people are saying about properties.
After you have reservations in Venice, he sure to get specific directions to the property--including where to get off the vaporetto (water bus.). And remember to travel light as everyone will need to handle their own luggage--that often requires getting up and down bridges.

For the short amount of time that you will be staying in each city, I would recommend a hotel rather than an apartment. You will probably have to get two rooms, but I think it would be far less of a hassle than renting an apartment for short period of time. With a hotel, you will have someone to ask questions of and breakfast is included.

I also agree that 3 nights may be a bit short for apartment renting. Too much time goes into the checking in and out process with apartments, because you have to be walked through the functioning of things and appliances, and also because you have to clean up a bit and leave things properly before leaving the property. Also many apartments require a minimum number of nights which could be longer than your stay, so choices will be more limited.
There are hotels that have quad rooms or mini suites that can accommodate 4.

Another opinion that for three nights you will be better served with a hotel. With regard to safe and convenient areas, Venice is quite small so everything is convenient. The main places you'll probably want to see are also in a fairly compact area. Rome is a bit more spread out, but (with some research) is quite navigable. We've never felt insecure in any of the three cities even at night. Just exercise the normal precautions you do in any large city and you should be fine.

I couldn’t disagree more with the above posts. Renting an apartment for four people will provide more space and comfort, for a cheaper cost, than any hotel in popular cities like Venice and Florence. Our family of four has stayed in a number of AirBNBs and check in and out is simple- usually there is a lock box with the key inside. The owner provides the passcode to get out the key. You return the key to the box when leaving. We like to rent apartments with washers (and dryers when available) so we can pack light and wash clothes as needed. Clean up at the end is easy - and if you have been keeping up, should take no longer than 20 minutes.

And I don’t get why the length of stay would matter? I have waited longer in hotel check in lines than I have walking into my rented apartment. As for breakfasts, we prefer to go out in the mornings to local cafes and really experience the atmosphere - especially in cities like Venice and Florence with all the fantastic coffee and pastries. Sometimes we bring it back to the apartment and sometimes we are out early to see the sites.

Often, we tire of the constant diet of restaurant food and will stop by a deli on the way back to the room and pick a few things to eat in the apartment - sometimes with an inexpensive bottle of wine or local sparkling water- all for the fraction of the price of another dinner meal.

I am currently planning a trip to Venice and Florence and have seen some amazing apartments on AirBNB - especially in Florence. But book early- many are already reserved for April.

Side note- just reread my post and realize I sound like an ad for Airbnb - lol- I am not a host- and I know there are other apartment renting companies around.

In Venice, we like the Canareggio area. A little quieter, more residential, less touristy. Still very easy to access all the sites.

We never stayed in Florence, we stayed in Siena and day tripped to Florence (I know, I can hear comments now, but I can only take so much art!).

In Rome, we previously stayed near Termini, which was ok, felt safe, but this time we are staying in Trastevere, which looks much more homey. Comes highly recommended on this site.

We have never stayed in a hotel per se. We stay at bed and breakfasts, and Airbnb. You can find anything from a single room to an apartment to a full house on Airbnb.

In Florence anywhere in the historical center works. The Oltrarno neighborhoods (south of the river) are generally a bit less crowded.
Plot a one km radius circle from Piazza della Repubblica, and that will put you in the middle of everything.
https://www.freemaptools.com/radius-around-point.htm

One thing to consider with an Apartment or an Apartment type setup is how to get the key and how to return the key. With a traditional hotel, I've found that quite easy and if you are leaving at a decent hour the Apartment may not be an issue, however, if your travel plans involve a very early departure be sure to clear that with wherever your rent from but most hotels readily accommodate that.

I would also chime in and say: rent an apartment if you want to!
I have rented many over the years in Italy, and only one wasn't up to my standards, as it was dirty and smelled like cat pee in the kitchen; we had to clean it all before we settled in.
Otherwise, I have had marvelous hosts, and lovely apartments every time.
You'll have a washing machine, a kitchen, and hopefully a terrace ( 3 out of my 4 criteria for apartment rentals, along with Wifi.)
You will get the Italian experience in a deeper way then a hotel stay.
It's fun to pretend you live there too!
Look very carefully at all the photos, and be aware that sometimes a "second" bedroom is the living room, though not always.
Just look for ones where the host meets you in person, so you don't have to go looking for where to pick up keys.
In Florence, I like the Oltrarno side of the river, not so noisy as the Historic Centre.
In Venice: rent within a short walk of the train station, as there are many bridges for you to walk up and down.
Canareggio and Dorsoduro are easy to walk to.
And pack light!
Also: be aware that in apartments, the heat may not be turned on at that time of year yet; there are laws regarding this.
Not sure of exact dates for turning on heat in Italy, anyone else know?
So if you need to be warm, a hotel would be better in Venice.
I was there last September; it rained a lot, and our apartment, though perfect otherwise, was a bit chilly and damp feeling.
Happy planning!