Welcome to the France forum.
In order to get the most comprehensive replies, please provide additional information.
--What are the dates of your trip, including arrival and departure?
--Are you planning to rent a car or take trains to get to other destinations after Paris?
--Is this your first trip to France?
--To give an idea of your budget, what are you paying for hotel rooms in Paris and Nice?
--Does the "we" consist of two persons, or are more than two persons included?
3 days in Paris is not much. I always recommend a bare minimum of 5 days just to scrape the surface. Why not stay in Paris longer? If you are on a budget, you can rent an apartment vs a hotel. If you are looking for a town between Paris and Nice...there is Lyon. There are also plenty of beautiful towns near Nice. I think every one could help you more, with more information.