I just spent 10 days with my teenage daughter in Paris,,,walking and taking the metro everywhere. Also went solo a few years back,,,

3 Days will not afford you much time to walk around to everything so stick to the main sights along the Seine. Go back when you have more time AND with a partner.

Along the Seine, you can see most of the major monuments and museums without having to figure out the metro. The Left bank is for "thinkers",,,the right bank is for "spenders"!!

St. Germain has wonderful Cafes' and restaurants. The Sorbonne area is full of young college hangouts and relatively inexpensive food (compared to the Opera QTR/Fauchon/Lauderee,,,et al.

Here is my list of "must do" in a short visit

Sit in a Cafe/Cafes
Rue de Bucci Morning markets
Rue Moufttard Morning markets
St. Germain
Notre Dame Cathedral
Saint Chappelle
Opera Quarter
Arc de Triumphe
Tour Eiffel
Museum D'Orsay
Tulleries Gardens
Versailles