I am looking for information about doing a day trip from Paris to Loire Valley in particular Chateau de Chenonceau, Cambord and d’Arzay-le-Rideau.
Is it possible to view 2 out of 3 of these chateaux in one day without a car?
How does transportation work?
I heard that Tours tourist office offers minibuses to visit the châteaux. Has anyone done that?
Thanks for your help in advance!

Yes via the mini-bus tours that visit 2-3 castles on full day tours - best to do is hop TGV to Tours station and then mini-buses from there. (ACCO-DISPO is one such company that's been around a long time) - the Tours tourist office i think books tours for private companies like above but doesn't run their own tours. Mini-buses take several folks on a circuit - some include admission fees some don't. Tours tourist office should have all the details and you could check when you buy your train ticket in Paris about mini-bus tie ins with the TGV tickets.

If you're not averse to driving (and the driving is easy in the Loire Valley), you could easily visit Chambord on your way in from Paris (about a two hour drive) and then take in Chenonceaux, and go on to Azay-Le_Rideau. There's a lot to see in the countryside, and if you control the transportation, you won't be rushed. Those three, however, make for a long day, since there's a lot to see in the chateaux and on the grounds of Chambord and Chenonceaux. Azay-Le-Rideau is beautiful and fun to walk around, but it takes less time for a visit. You might (with or without a car) consider Chenonceaux-Amboise (lots of places in the town for lunch) and Azay-Le-Rideau as a more manageable threesome, especially if your start point is Tours.

My mom and I took a one day guided tour on a small van. 7 tourists and a guide/drive. I can't remember now if we booked with Cityrama or Parisvision, but my recollection is they were basically the same. We saw Chambord in the early morning, then Cheverny, a lunch stop, then Chenonceau. It was great. I really liked having someone else do the driving, and I don't know if I would have seen 3 if I had done it independently. Our guide was excellent. And I felt like we had enough time in each place, some time guided and a bit of time to wander on our own. The only time I was sorry not to have more time was when we were leaving Chambord. They were just setting up a market with lots of local food stuffs and wine, and we didn't have much time to stay and check it out.

I highly recommend the tours.

We also did the http://www.accodispo-tours.com tour, contracting the whole van for 7 of us. We had flexibility because of that, going where we wanted. We took the TGV there (Tours?)and a slower one back. They are essentially drivers who will help you buy the tickets at each castle and you tour on your own. It beat renting 2 cars and the logistics of driving around.