We are trying to reserve a couchette from Biarritz to Paris CDG for an 11:00 AM international flight. The train schedule shows that the TGV ends at Montparnasse at 6:00 AM. How do we get to CDG? How much time would we have for all the connections? Would you risk it?
I would risk it. Three hours is (figuring arriving at CDG 2 hours early, which may be generous) is plenty of time to get from Montparnasse to the airport via public transportation.
A bit of a slog home, but very feasible.
I posted a similar question and a helpful reader send me a link to this fantastic website to figure out the public transportation options. http://www.ratp.info/informer/anglais
I looked and it'll take you about an hour. Set your start search term as Montparnasse-Bienvenue-Metro (this is the metro station just outside the train station). I was a bit confused on the end search terms. I found a metro station called Charles de Gualle-Etoille, but I don't think that gets you close enough to the airport. I found a search for Aeroport Charles de Gualle 1-RER that I think gets you all the way into the airport.
Hope this gets you closer. I do know that with your 3 hour timeframe (to get to the airport by 2 hours before your flight), you have more than enough time.
There is also a direct Air France coach from Montparnasse to CDG. This is much more convenient than the metro/RER. Check Air France's homepage and click "information and services" then "Practical information" then "Air France Coaches". Journey time is approx. 50 minutes, departure every half hour, price 14 € o/w.