I just got back from Florence and we stayed just off Piazza della Repubblica. I think Florence lends itself to being in the middle of things. I'm surprised some think St.R is a bad location - it's barely 10 minutes walk from PdR / Duomo/ Vecchio.
JK might be a bit close to the train station for me (my main issue with StR Rome). FS is too much walking if in town for a only a couple of nights IMO.
Florence I'd probably do Portrait.

Rome my clients like Fendi, Portrait, JK or Hotel de Russie (yes its flawed and overpriced, but I like it and the location).

Venice - Aman or Gritti - whichever location appeals most (like Rialto, everywhere around Gritti can be busy, but both area pretty much equal distance from Dorsoduro which is my favourite area of Venice).