I would suggest taking the train simply because there is no need of a car in either Venice or Florence. Rimini is small and quite manageable. (Although you would need a car if visiting San Marino). The train journeys from Venice to Rimini and Rimini to Florence would most certainly involve a change in Bologna. www.trenitalia.com is the official website for train services. Hope this helps but if you need further information please do get in touch.

Regards