I'm looking for suggestions for a 3 star hotel in Paris city center, it needs to be within a couple blocks of the RER B (to take to the Paris-Norde villepinte exposition center).

rather than stars, can you say what you'd like to spend per night? Single or double? A/c needed (in season), or not?

ttt

I'd recommend one around the Luxembourg stop, I think that is the most pleasant. Here are a few in the 4th arr. near that stop:

Hotel des Grands Hommes
Hotel du Pantheon
Hotel Jardins du Luxembourg
Grand Hotel St Michel
Hotel Select
Hotel l'Observatoire

There are also a few in the 6th not far:
Hotel Luxembourg
Hotel Trianon Rive Gauche
HOtel St Paul St Germain is just a bit farther from those, but not too bad.

If it were me, I'd look at the Pantheon, it's always sounded nice to me. www.hoteldupantheon.com
The Grands Hommes is same owner and nearby.

FYI, just a typo, my first list was in the 5th arrondisement. you can get addresses and some info on www.paris.org/Hotels as well as Tripadvisor

Look into the Henri IV Rive Gauche on Rue St. Jacques in the Latin Quarter, just one block south of the Seine and Notre Dame. The RER B line is St. Michel/Notre Dame and is also one block, straight up toward the Seine from the hotel. 3-star.

www.hotel-henri4.com

The Hotel Select, on the pedestrian Place de la Sorbonne, is an excellent choice. I stayed there last week for 3 nights and highly recommend it.

My suggestion for a short walk from the Luxembourg stop is the hotel Delavigne, as long as you don't need a/c. I stayed there last January and really liked it.