I am arriving at CDG at noon. Where should I drive the first day to start a 2-3 day visit to Loire Valley

We took the train to Chartres and picked up our car there. Then visited the cathedral, walked the town a little and drove on to the Loire. Our b & b was in a small town several kilometers from Amboise. The plan worked very well. Enjoy your time there!

From memory, the drive from Paris to Tours is only a few hours, really not very long, though I'd suggest inputting it into viamichelin to check, so you could hire a car at the airport and get right down into the Tours area for your first night. In that area you will find lots to see such as one of my favourites, Villandry.

Have you already made hotel reservations or are you actually asking what town you should start out your visit in?

no reservations. Wondering where to stop on a first night, short day

The Loire is full of small private chateaux-hotels - many of which are charming and quite inexpensive. As long a you have a car I would build the trip around one of more of these.

A couple of years ago we stayed in one year Chinon (the drive from the arrport was about 3 hours I think) and we could easily see everything we wanted from there - as well as have the experience of staying in a 15th century fortress.

http://www.bienvenue-au-chateau.com/

This is a great site for accommodations in historic buildings in the Loire region. We've stayed in several over the years and not been disappointed.