As you are only scheduled to land at 12h55, no one know exactly how much time you will actually have to reach Montparnasse. Assuming you are checking luggage, I would allow 30 minutes to deplane and collect luggage, Immigration can take anywhere from 10 minutes to 1 hour+, the official taxi queues are just steps from the Customs exits. There is never any delay at Customs.

Processing at CDG can take anywhere from 45 minutes to 90 minutes. There have recently been significant delays a Immigration, both inbound and outbound. You could make a 15h55 Montparnasse train but I would not attempt the bus, take a taxi. The price is fixed at 55€.

Overall, 3 hours is not sufficient time to guarantee your making a CDG to Montparnasse connection on any consistent basis.