Looking for a relaxing spot for 3 days in late July on the Mediterranean Sea

Ramateulle is on a hill above the beaches, near St-Tropez.

I'd think it would depend on where you'll be going after this adventure. But I love Collioure and, to keep travel to a minimum, I think I would look at Perpignan.

Why not just spend that time staying in Collioure.? Easy train trip from Perpignan. It is a beautiful little seaside town. Has plenty of good affordable accom. We stayed at the Templiers.
We spent a few days there in July and loved it. It is a bustling little town in summer. Plenty of things to see and do.
I went swimming at a number of different beaches in and around town.

Albi can be shockingly hot in the summer (as can Toulouse), and I think it would be very time-consuming to make trips to the beach if you used it as a base. It's quite far inland.

Collioure is picturesque but I found it too touristy to contemplate spending multiple nights there. That's a very personal reaction. I'm also not sure it will be any less expensive than places on the Riviera.

Perpignan has points of interest, but I suspect you'd prefer a place with a bit more charm for a three-day stay. However, the city is a hub for both trains and buses, so moving around would be easy. You could even take a trip on the Yellow Train into the Pyrenees. And it was pleasantly non-touristy in June 2017.

The university city of Montpellier is worth looking into. It has a very nice historic area, and you should have reasonably quick access to beaches. (I am not a beach person myself, so I don't know what any of the French beaches are like.)

I also suggest investigating Narbonne. It's large enough to keep you somewhat occupied for multiple days and has good rail service.

I liked the small town of Pezenas, which is known for its crafts, but it might be too small for three days, and the coast seems to be more than an hour away by public transportation.

I think Arles or Aix could work (Nimes is a bit farther from the coast, and I haven't been there), but they are very popular destinations.

I'm not a big fan of Albi. I liked the church but the town wasn't my favorite. Perpignan seems more of a connection for transportation than a destination, look at Narbonne as an alternative. To the north, I think I'd be most happy spending a few days in Sarlat. Nimes probably has enough good sights for a day. I really liked Arles as a base for Provence. For three days, it might be a really good choice. Once you're there it's a walkable town with quite a few sights.

Since you will be ending near Toulouse, it makes sense to drop down to the beaches of the Languedoc.

Perpignan is not my favorite at all, and if Coullouire is too touristy, be warned that many beaches that are easily accessible by transport are also going to be crowded.. Some of the beaches just south of Narbonne can be less than appealing -- ugly concrete holiday flats and strips of not so nice restaurants. Leucate is good but I think you need a car to get there. The sand and water are great, however, throughout the region but wind can be an issue.

Consider also the suggestion of Montpellier. There is direct public transit to the beach from the center, but it is going to be popular (and a couple are filled with ugly blocks). Espiguette is a one of the best beaches and is close to Aigue Mortes. Adge is not a bad town but it's also adjacent to a huge naturist resort but nearby beaches are good -- nearby Sete is great as well but may be too touristy. South of Bezier there are also some nice beaches, and the town is much smaller than Montpellier.

Thank you mes amis pour votre good advice! We have decided to stay in Montpellier, at the Hotel du Palais, and take day trips on the bus to various beaches and nature preserves. Your advice was ever so helpful!!

Ruth Little