You've already had a number of well informed opinions, but for what it's worth:
Rome: St Regis. The other luxury hotels in Rome all have something seriously wrong with them, IMO. This will not be the best hotel on your trip, but such is Rome
Venice: You're right to pick the Gritti.
Florence: While I think that the St Regis Florence is perhaps the best hotel in the Marriott portfolio globally (at least for service), it isn't on quite the same scale of grandour as the Four Seasons with their lovely gardens. Pick based on whether you'd like a luxury boutique great service feel (St Regis), or grand luxury at the Four Seasons.