Versailles is wonderful, disagree with the poster above. The grounds, and the Petit Trianon are my favorite. You can rent biles there which is fun. March is a great month to go, it will not be packed with tourists. It’s well worth the time but with only 3 full days i wouldnt make it a priority unless you love the history of it.
The best way to experience Paris, imo, is not the tourist sights and not the museums, it is walking and exploring. Along the river, the 2 islands, through wonderful neighborhoods and parks (especially the Luxembourg Gardens). That is the best of Paris imo.
That said, we went up the Eiffel Tower two weeks ago (have done it 12+ times) and it’s very fun.
The Opéra Garnier is wonderful but the front is covered in scaffolding, getting ready for the Olympics and being on the world stage. Lots of scaffolding and cleaning going on in Paris now.
Agree with Place des Vosges, it’s beautiful. Carette at Place des Vosges is one of my two favorite cafés.
I am not a fan of Montmartre. Save your time for exploring along the Seine river and the Marais, St Germain, Latin Quarter areas.