Any place within the historical center of Florence or Venice will be close enough to the main train station.
Milan is bigger and has 23 train stations, 3 of which are major, but anyplace in the city center will also be close enough to the main stations of Milan either by foot or by subway.
The cost of taxi varies on the distance traveled, the traffic, and the time of day. In Florence the fare is €1.10 per km while in Milan it is €1.28 per km. But then there are flag down charges, and other supplente (like luggage).
Venice has water taxi boats, which are very expensive. But you can use water buses, called “vaporetti” (literally “small steam boats” which actually run on diesel) and cost a little less (about $10 per ride, but there are day passes).