3 nights Paris towards Barcelona via train--where/what recommended?

One answer is that there is more than one route from Paris to Barcelona.

There is the TGV route which everyone knows.

Option 2 is the day or night train to Pau (or Lourdes), then a French TER train from Pau towards Canfranc in Spain (part replaced by bus for the final few miles to Canfranc- you struggle to find this route in most journey planners). A new luxury hotel has just opened in the former magnificent Canfranc Railway Station. Then local train to Zaragoza for the final high speed leg to Barcelona.

The third route is to Hendaye (day or night train) on the French/Spanish border. The night train conveniently misses the day train to Madrid (change for Barcelona) by a few minutes. However you can take a RENFE Cercanias (commuter) train (not in RENFE journey planners) to Donostia/San Sebastian for other fast trains to Madrid.

The fourth route is explained (but not very well) by Seat 61- https://www.seat61.com/trains-and-routes/paris-to-barcelona-by-sleeper-train.htm
Paris to Toulouse by day or night train, then Toulouse to Latour de Carol. On certain dates the night train extends to Latour. On dates when it doesn't there is a connecting day train, with a 2 hour connection at Toulouse.
Day Trains from Toulouse to Latour run roughly every two hours.
Then a commuter train (again not in RENFE journey planners, as explained by Seat 61 above) to Barcelona.

From Latour de Carol there is another much slower option (route 4a)- the scenic and famous Petit Train Jaune to Perpignan for main line trains to Barcelona.

So you are looking for two nights and then Montpellier. I suggest Dijon the capital of Burgundy and then Vienne just south of Lyon. Vienne was an important Roman port city on the Rhône, while Dijon has a rich history from Medieval times forward.

The Paris to Montpellier TGV itinerary stops only in Valence but out in the countryside, and Nîmes center city, which it is only a 20 minute train ride from Montpellier.

You can get a TGV from Paris to Dijon at the Gare de Lyon, or take an excruciatingly slow local train at the Gare de Bercy.
From Dijon to Vienne, you change trains in Lyon. Once south of Valence, the Montpellier train does not veer off into Provence, so I haven’t suggested anywhere in Provence.
If you want an easier stop than Vienne, you could stay in France’s second largest city, Lyon, and catch a TGV from Lyon to Montpellier. Your easiest route from Vienne to Montpellier is to either backtrack to Lyon to catch a TGv, or see if a lical train from Vienne takes you to the Valence TGV station. SNCFConnect will give you the routing.

Or, spend three nights in Montpellier. It’s 3 hours to Paris and 3.5 to Barcelona. The Renfe train leaves around 9 in the morning from the Montpellier Saint Roch station.

Or you have the routes above that Stuart suggested, everything except walking with a donkey through the Cevennes mountains.