This is the most direct way to reach the dunes of Erg Chebbi from Marrakech in 3 days and return by road. The route crosses the High Atlas via the Tizi n'Tichka pass, follows the kasbah route through Ouarzazate and the Dades Valley, and reaches Merzouga on the second afternoon.
It's an itinerary built around the road. Two of the three days involve long stretches in the vehicle, and the third day is almost entirely the return to Marrakech. If your priority is watching the landscape change from mountains to pre-Sahara to dune field, this route delivers exactly that. If you'd rather have calmer mornings, the four- or five-day version is a better fit.