You need a car to do this.
You can go to the website viamichelin.com and get info on drive times and distances, toll and fuel costs and suggested routes (i.e. scenic routes). The drive times given do not consider stops (fuel, food, bathrooms) nor do they consider bad weather and bad traffic. I find the drive times very accurate when these factors are accounted for. The time estimates can break down when you are driving in congested urban areas, like in or near Paris, due to the unpredictability of heavy traffic or traffic jams. They can also be affected on peak travel days, specifically on autoroutes leading to/from popular destinations.
Normandy is a region, not a town or city, and Etretat and Mont-Saint-Michel are IN Normandy. You can not visit Etretat and MSM on the same day if starting from Paris. If by Normandy you mean the D-Day landing beaches near Bayeux then to do Etretat, D-Day sites and MSM (assuming you rent a car and start from Paris) you need three days and two nights. This would also allow you to see a bit more.
Day 1 - Leave Paris and go to Etretat then head to Honfleur and spend the night.
Day 2 - Visit D-Day sites and the Bayeux Tapestry Museum.
Day 3 - Visit MSM and go back to Paris. You could drop the car off in Rennes, spend a few hours visiting there and take the train back to Paris.
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