Yes, reservations are generally recommended in May
Ciao Patricia,
I can understand you wanting to remain flexible and wait until you get to Tuscany to decide where to go and where to spend each night but hotels, B&Bs and farmhouse and villa apartments here are not set up to work that way (I think it is only hotels that generally have software that automatically update their availability across various portals so that you can see last minute availability online so at the very least you'll need to have a smart phone with you). May is not high season but it is already busy... and, I assure you, there are no signs outside hotels or other lodgings that display a "rooms available" or "vacancy" notice. Once you're in a small village, how will you know where places are, particularly if they are in the countryside outside of the village? how will you know whether they have a room? you won't want to waste time going from place to place. Do you really want to use up the limited time of your 3 days in Tuscany worrying where you'll be spending the night?
As far as I know, only Florence (and larger towns such as Rome, Venice) has an office in the train station that helps find last minute availability at local hotels, most other towns and tourism offices are not that well organized... so my recommendation would be to decide some general areas you do want to visit each day and pick a location where you want to end your day at to and book ahead of time, it will save a lot of headaches! Take a look at
Tuscany Accommodation to start your search.