good times! I also did a 3-week climbing trip thru France a few years ago. I still have several guide books and this cool map of France that shows the size and location of climbing areas all over the country. you are welcome to borrow it all, if you do please treat the material well and return it at the end of your trip.
I would second some of the recommendations here: climbing in Les Calanques and Orpiere, visiting Cassis and Chamonix. as was said before dont plan on climbing in Cham that time of year, like here, the weather can be a mixed bag in the mountains. plan on hitting the places that will most likely have good weather and then once you get there modify your plans to go where the weather is good. like HC said you can travel anywhere in a day so just be flexible.
if you have any interest in alpine climbing history you should at least visit Cham even if you dont climb there. and as it is full of Brits and just about every other nationality you can easily get by speaking English so it doesnt sound like what you are looking for in a climbing destination anyway. but visit for a day and eat great fondue!
if you are still looking for reco's I can give you more for other areas. not sure what you are looking for in your climbing, we were looking for lots of sport climbing 5.10 and under on that trip. I've been there on other trips for alpine climbing, Haute Route, etc. so we can talk about that too if you are interested.
hope some of this long babel helps,