I am asked all the time for help with itineraries to France. If you've been before, great! I can recommend some wonderful "off the beaten path" locations. But for the first timer, I will start by admitting there is no perfect itinerary. And, follow with, 3 days is not enough time in Paris.
That said, I offer the following very ambitious 3-Day itinerary for Paris. It assumes you will not do everything, and that this is all the time you have for the City of Lights!