Well, trying to deviate a little the discussion from Siena and Cinque Terre, but focusing on a more practical point of view, I would probably start with Rome for two reasons:

a) More flight options, and
b) Rome hotels are more expensive than either Firenze or Venice, so I would try to book a plane+hotel package, probably using Expedia for the booking.

Doing this you will probably have some pretty good savings, since Rome hotels are pretty expensive.

From Rome, train to Firenze and than train to Venice.

I'm not sure that Rome hotels are more expensive than Venice. For Rome, I can recommend Pensione Barrett, which is Michelin recommended and under 110e for a single and incredibly well-located.

http://www.pensionebarrett.com/Serviziuk.htm

Before booking a hotel/flight package should you find a more attractive rate, be sure to use a google map to ascertain exactly where the hotel is. The historic core of Rome is not large, but Rome itself is, and some hotels -- especially the corporate-owned chain hotels with steep discount rates -- are located in Rome's sprawling modern periphery.

I think the summer heat is getting to everyone. not just this post. Time to relax take a deep breath, and think what is this all about.

luckytotravel: <i>So even after reading all of this i am not sure how easy it is to visit Cinque Terre on a day trip from Florence.</i>

I have no idea if the original poster has even the slightest interest in visiting the Cinque Terre on this trip or deviating at all from the stated itinerary of Venice, Florence, Rome. I just threw out the suggestion. I heard about the CT years ago but before my first trip to Italy in 2007 a friend asked me if I was going there and I'd forgotten about it and put the CT in my itinerary - and I'm SO glad I went. Some people may never have even heard of the place. This wouldn't be everyone's choice, but if I'd had only 11 days in Italy, I'd definitely have wanted even that one quick day-and-night up to the CT. (I had more time and did three nights on my trip.)

But I think it's quite easy to get to the CT from Florence. That's how I did it. As noted, it's about 2:30 one way on the train if you catch the right one. I also returned back to Naples from the CT in a day - a long train trip but doable; returning to Rome from the CT (if that's how the itinerary would play out) would be an hour or two less and quite reasonable. Depends on a traveler's tolerance of train travel I suppose - I enjoy it.