The
Bus Direct to Montparnasse first stops at Gare de Lyon. The ride can seemingly take forever to reach Montparnasse. At 3 x 17€ the cost for the Bus is 51€. A taxi to Montparnasse from CDG will cost a flat 55€.
The RER B is 10€ each but will require an awkward connection at probably Denfert Rochereau. If you have a lot of luggage, I wouldn´t attempt it, particularly at rush hour.
No one option can simultaneously be
best/fastest/cheapest. In your case however, a taxi, from the official taxi queue, will come close. Do not accept offers from clandestine drivers soliciting from within the terminals.