Hi Talon,
Terminal 3 is really close to the Roissypole train station. Take a look at the first picture on the
CDG Terminal 1 to Paris by train article. Terminal 3 is top center. You'll simply walk to Roissypole, directly south of Terminal 3.
The big confusion with the location of this train station is due to its name: Aeroport Charles de Gaulle 1 - RER. As you can see from the first photo/map, the train station is not at Terminal 1, but rather, at Terminal 3 and the
CDG Roissypole building seen here.
Now, as for Argentan, you'll be taking a Corail or Ter "branded" train from
Gare Montparnasse. (The type of train is not so important, the ticket windows at Gare Montparnasse are generic and sell tickets for all various "brands" of train). The train platforms at Gare Montparnasse are found at Hall Pasteur, 2nd level up.
If you know exactly when you'll be traveling, you can purchase a train ticket from Paris to Argentan for as little as 10€ from
TGV-Europe.com. A regular price train ticket, bought directly at Gare Montparnasse is 25.80€.
I'm currently writing an article on using the TGV-Europe.com website for purchasing tickets, so keep an eye out for that if you find the site confusing.
Hope this helps,
Ben