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.
http://parisbytrain.com/wp-content/uploads/2008/07/aeroport.pdf
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.
Your best bet using Paris transportation system :
Reach the station located in terminal 2.
Buy your ticket to Paris at the green machine if you have coins or a chip-and-pin credit card. Else you have to queue at the "regional" ticket booth, not the "grandes lignes" which is for long distance high-speed trains. A ticket to Paris is good for any destination inside Paris, including metro lines.
Take the RER B at the dedicated platforms. Although there are several branches and express/local services, all trains serve all stops inside Paris. Therefore you can board the first one available.
The train stops at several stations inside Paris. Get off at Denfert-Rochereau.
Using the same ticket, transfer to the metro line 6. Follow the "Etoile" direction (westbound). Get off at Montparnasse-Bienvenue 3 stops later.
Follow the "Grandes lignes" directions to reach the Montparnasse train station. After you pass through the exit gates, take the sideways escalators up to get to the long-distance lines; or those in front of you to reach the commuter platforms.
Do not use line 4 for this trip, if possible. Line 4 serves Montparnasse and Denfert-Rochereau too, but its platforms are at the far end of the metro station; it would require you passing through several flights of stairs and the locally famous moving walkways for 6 minutes of walk at least and stays rather impractical when carrying luggage. In contrast, line 6 platforms are the closest to the train station.
Sometimes, when getting off the last evening TGV, you can see people running really fast while hauling their luggage through the metro station although it is 1am, since they hope to get the last line 4 train. By the time they reach the platforms, it is common that the last train just went off!
The RER B line does not go directly to Gare Montparnasse. You would have to transfer to the metro at Les Halles or Denfert Rochereau (or possibly a couple of other places, see the metro map). You could transfer to metro line 4 as early as Gare du Nord, but the RER is much faster than the metro when going through the city.
