I'll stay at Paris for 3 hours for a connecting train. What are the places I can visit by this time?

Well, as one person pointed out three hours isn't much. And to my knowledge, for security reasons it's no longer possible to store one's luggage in a locker (though I could be wrong about that). If you're coming from London, that means you'll arrive at the Gare du Nord. You might want to consider just finding a nearby café and killing a bit of time there. If you had more than three hours, and if it were possible to put your baggage in a locker, I'd of course recommend something as simple as going for a walk along the Seine (easily reached from the Gare du Nord by taking métro line 4, direction Porte d'Orléans, and getting off at Châtelet or, better yet, Cité).

Take the subway to the Trocadero - there you can see the Eifel tower, les Champs Élysées, les Tuileries and the Louvre. You won't have enough time to visit those places, but you will be able to see them. It is a lovely walk from the Eifel Tower to the Louvre. And from the Louvre, you can walk to Ile de la Cité and see Notre-Dame. Three hours in Paris is not a lot of time, but you can still see some of the most famous sites.

If you wanna do something very special? Go in front of the railway station to : LE ZINC this is for me one of the most typic "bar/tabac" of Paris and ask for " un coup de rouge" Just do it it´s not only good but the most important L´ambiance de Paris et de ces titis...... Enjoy it!

3 hrs is not that much. It all depends which train station you will be arriving and/or departing in Paris.