Recommendations for a place to stay in France between Barcelona and Paris

You could take a train from Barcelona to Avignon and then go from Avignon to Paris after your 3-day stop. We have taken the train a few times between Barcelona and Paris. The trains we have taken stop in Nimes (also a potential stop) but there is at least one direct train from Barcelona to Avignon and a few single-connection routings and then later in your trip, you can get a high speed train from the Avignon TGV station to Paris. Avignon is a good spot to base yourself if you will not have a car because you can use it as a base for trips via mass transit to other spots in western Provence or even to Languedoc-Roussillon to see Pont du Gard.

Well, if you have 3 days to utilize, I'd consider the Loire Valley. Except for this bit...

We won't have a car and will be relying on train service and taxi

Getting around the Chateaux is best done by car (not impossible to do without a car, but not nearly as efficient or pleasant). If it were me, I'd get off the train, rent a car for those few days, then drop the car and take the train back to Paris.

The Loire Valley is a lovely idea but getting there by train from Barcelona is usually going to entail going to Paris and changing train stations. It is possible to take a train from Barcelona to Valence and change there for a train to the Loire Valley but I think there is only one such train per day. If you want to visit, the Loire Valley and will not have a car, I suggest basing yourself in Tours.

I second the recommendation for Avignon, but nearby Nîmes is also a lovely place to stay and it's more convenient coming from Barcelona. Perfect for three days, you can visit Nîmes itself, the Pont du Gard, and Arles or Avignon for instance.

I'm guessing that when you refer to a "3-day stop" you mean 3 nights. The direct trip takes 6-1/2 hours, so 3 nights along the way means not much more than days of sightseeing time. Accordingly, I'd prioritize places with sights of their own or with a single really good side-trip.

The fastest trains between Barcelona and Paris are TGVs departing at 9:25 AM and 1:20 PM. These are the French stops along the way:

Perpignan
Narbonne

Beziers (9:25 AM only)

Montpellier Saint-Roch

Nîmes (9:25 AM only)
Valence (1:20 PM only)
Paris Gare de Lyon

You could change trains along the way and have access to additional cities, of course, but that will substantially increase the time spent on the train. Going to Avignon (which I really enjoyed) would increase the transit time to just over 8 hours even if you never set foot outside the train station. Arles (also nice) would mean 10-1/2 hours on the train.

Off the direct rail line is Lyon (7-3/4 hours), but it is perhaps not ideal in this situation, being wonderful but perhaps a sort of mini-Paris.

Another possibility, but requiring significant time on local trains and probably better for those having more time, would be the popular (i.e., touristy) small coastal town of Collioure. To me it's sort of a half-day place; it could be combined with Narbonne or Montpellier.

Of the places along the direct rail line, I've only been to Perpignan, Narbonne and Montpellier. Based on those visits and what I've read on this forum, I think the most attractive option for a short visit would be Narbonne, Montpellier or Nîmes. They each have their own advantages, so it depends on how you most enjoy spending your time.

"Going to Avignon (which I really enjoyed) would increase the transit time to just over 8 hours even if you never set foot outside the train station."
I respectfully disagree. There is a direct Ave train that makes the trip in four hours and a couple of routes through Nimes and Narbonne that take around 5 hours.

A third vote for Avignon here, but also agree that Nimes is also very nice & much easier to do via the TGV.

Carcassonne is one of the nicest places I've stayed in France. Stay right in the medieval walled city. Its touristy by day, so use that time to explore outside the walls, but by night its magical.

4 1/2 hours by train Barcelona-Carcassonne (connection in Perpignan). 5 1/2 hours Carcassonne-Paris (connections in Narbonne, Montepellier).

JHK, I meant that the travel time all the way from Barcelona to Paris would increase to 8 hours, not that it would take 8 hours just to get to Avignon.

Ah, I see. Thanks for the clarification.

Thank you all. I can't tell you how much I appreciate everyone's suggestions. I will have fun researching the different places and expanding my knowledge of traveling in France.

I googled Nimes and oh my, what a beautiful city with the most amazing Roman architecture everywhere, right in the (south) middle of France. Thank you for bringing that lovely place to our attention and for the great map of all the train stops between Barcelona and Paris.