Driving from Arles to Monte Carlo in 3 days

I'll assume you have three days / two nights excluding nights in Arles and Monte Carlo and excluding time to visit their immediate areas.

With this assumption you have two broad options: inland, or coastal.

The coast is very busy in August but you could for instance spend one night in Cassis and one in Sainte Maxime (a ferry ride away from St Tropez). Places to see and stop at along the way include Cap Canaille cliff near Cassis, Sanary sur Mer, le Rayol botanical Gardens, the coast drive between St Raphaël and Mandelieu- list not exhaustive !

Inland, which has my preference, involves going via the Verdon Gorge. Day 1, drive to Moustiers Sainte Marie with a stop in Aix (confusing to drive in, but easy parking). Day 2, explore the Gorges (scenic driving, hiking, boating are options), stay in Moustiers again. Day 3, drive to Monte Carlo, ideally via Castellane and Grasse to drive the scenic Route Napoléon.
Accomodation in Moustiers can be scarce, Castellane, Aups, and Aiguines all are alternatives.

Many thanks! We have three full days/nights.

With a third night on the inland route, I'd spend a night in/near Gourdon, Bar sur Loup or Tourettes sur Loup, beautiful villages in a dramatic setting and less touristy that Saint Paul de Vence (which is also an option) a bit further east. Doing so, driving from Moustiers I'd make sure to detour by Bargême, a very inspiring old hamlet that was nearly abandoned in the 1950s, then brought back to life.

Or if you want more action you can spend the night in Aix before Moustiers.

There are so many options...

We prefer coastal locations. Stop at charming Ramateulle on a hill overlooking the sea just before
St-Tropez. Beautiful wide sandy beaches below with a WWII landing history.
Next, stay in picturesque Antibes. Also consider Nice. So pretty with great dining and lots to do and see. We have been there twice, the last time for two weeks. Great public transportation up and down the coast. Next possible stop would be Cap Ferrat, so beautiful, site of Villa Ephrussi.
Honestly, our only disappointment in this area was Monaco/ Monte Carlo, huge high rises, marinas full of mega yachts, crowded, densely populated, expensive.

Aix is on the way. That may be a choice for one night.

I think a second good choice is above the coast, Eze or similar. Once you get down on the coast, you will want to lose the car.

If you stop at St. Tropaiz, I'd drop the car and stay on the coast by train.

Thank you everyone for all these wonderful suggestions!