I'd vote to do a number of things in the evening you arrive--which is lovely and lingering this time of year. Doesn't even get dark till 9:30/10pm, so I'd come into the city, catch a meal at a cafe, go to the top of the Arc (which may not have much of a line in the nighttime), or take a relaxing boat ride down the Seine and enjoy the lights. Then, get a nice petit dejeuner, stroll around a bit, pop into something with no line if time, and head out to the airport.

We almost missed our flight on AF coming back from Paris last month. First of all, we only allowed 2 hours plus from ND. Then, the RER train stopped on the tracks for 20 mins unexpectedly. When we arrived at check-in (we already had seats from internet) we were thrilled to see no line. At that point however we were told "the gate has already been closed", and there was no possibility of getting on. It was still 1 hour 10 mins before the flight time. We were stunned!

A little pleading on our part got them to pass us on to the next step with no promise of getting on, we were shuttled through priority-rush at the customs and passport control, and after jogging through the terminals, we just made it! At Dulles, where we left from, AF said to be there just 1 hour before and that was plenty. We also had our internet seats for the return, but 2 hours would be the minimum for AF at CDG, and 3 isn't unreasonable.

Enjoy your time in Paris!