How do I get from Charles de Gaulle Airport (CDG) to Gare Montparnasse
When I look at the map (linked below) it seems that I can go directly from CDG to Gare Montparnasse on the RER B3 line, but when reading posts it seems that many transfers would be needed.
Is it possible to go directly from CDG to Gare Montparnasse with out transferring trains? If not what path would I need to follow?
Given that you didn't mention a method of transport...
The easiest way to get from CDG to Gare Montparnasse without having to transfer is to catch the Air France Coach ("Les Cars AirFrance"), specifically coach number 4.
Update 2017/08 : Les Cars Air France is now Le Bus Direct.
Unlike attempting to catch the the train, these buses will pick up at all terminals at CDG (so there is no need to catch the airport train), and drop off directly outside Gare Montparnasse.
It will cost more that catching the train, and the trip time can be affected by traffic, but it's far more relaxing than the train, and without the need to transfer at least twice as the train requires.