3 days at the park? That sounds about right
For Paris, buy Rick Steves book and start reading. There's more in there than you'll ever see in 3 days, but a lot of ideas will spring from your reading. He's put together some short walking trips through some of the more popular areas. As E suggested above, focus on your interests and you'll have a blast.
Things I enjoyed:
Sacre Coeur in Montmartre
Pere Lachaise cemetary - Graves of Chopin, Jim Morrison, Moliere, Balzac and on and on and on
Museums - Out of the big three - Louvre, Orsay and Pompidou - I chose the Orsay for the impressionists and the smattering of Rodin. I knew I'd never get through all of the Louvre. My Orsay day was my favorite day in Paris.
Versailles? Could be, but it will take most of the day with the train trip out and back, the short walk to the palace and the line waiting once on site. Imagine you're viewing it through the eyes of a french peasant. I wouldn't expect to see the gardens or fountains. Even in the summer, the fountains are not on everyday, so I suspect during the winter they'll be on even less.
Boat ride on the Seine - Yeah, everybody does it. And you'll love it too. The boats at the foot of the Eiffel Tower are a pretty good deal. Get some hot chocolate for the ride. The city is beautiful at night from the river.
Enjoy!