Lorraine, the three locations are on the same street, Via Borgo Ognissanti (correct spelling). The one at 128R is Avis. By car, from Santa Maria Novella train station to Avis there are circa 2 km. For the other two locations, on the same street, I'd count the same distance.
To see it a bit clearer, go to Google Maps Get Directions, and key as follows:
Start address: Borgo Ognissanti, 128/R, 50123, Florence, Tuscany, Italy End address: Piazza della Stazione, 50123, Florence, Tuscany, Italy
Then click on "Get reverse directions". You will see that when going from the station to Avis the route is much simpler, and the distance is reduced to circa 1 km. The difference is caused by the one-way streets.
To walk, the second route is the obvious one, as I said above circa 1 km.
Finally, do not forget to read our discussions about the Zona Traffico Limitato (ZTL) in Florence.
At www.mappy.com , you can select an option for Pedestrian directions; they show number 128 (if they're really considering the r, which is another story) as 0.57 km walking from Piazza della Stazione. I hadn't used Mappy in a while, and the problem is that if I use the .com URL it defaults to the U.S. and I need to follow "Change continent" links.
Especially if you're planning to return the car in downtown Florence, note that, if I was right in another thread, the 53/55 address is on a ZTL-restricted section of the street and you'd need to be sure the office called to say your entry into the zone was authorized. If you're just picking the car up, you're at less risk of getting caught entering the ZTL, but there are special routes you need to take because of construction; search old posts.
You would do well to take a peek at the posting Got a Ticket, Now What for some cautionary advice about the ZTL, and pay particular attention to the excellent hand drawn map provided by itarchivarius. But do note his disclaimer; when we drove away from our rental pickup in August '08, local road work altered the route somewhat. If there is one thing you will learn about travel in Italy, "things change."
WARNING. Our first time in Florence and i also rented a car at the 100 R Via Borgognissanti they advised me it was the closest to the train station. It is not too far away but took us forever to find. The worst was dropping the car off, we could not find it again very easily. Finally we did get there, and when we arrived home in Vancouver, we also had 3 tickets for driving in the bus lane. It was all in that hour trying to find the car rental. My advice to you is if you can rent in a smaller place thaat would be better! Wilson
Here is a link to my trip report from 2004 about the two car rental agencies on Borgo Ognissanti for Auto Europe we visited on our trip. Due to a mix-up, we spent time at both of them. There was a huge difference in how they were run!