A great three-day run with challenging trails and incredible views. You'll enjoy the trip!
Day 1: Castérino - Refuge des Merveilles
The trail starts in the hamlet of Castérino. If you need accommodation here, I recommend Auberge Ste Marie Madeleine.
The trail winds up through the forest and passes the Refuge de Fontanalba (closed in 2017, but being renovated). Further on, the trail reaches its highest point at Baisse de Vallauretta (2270 meters). Here you have great views to Mont Bego and the area in general.
Then it goes down again to the road to Les Mesches, and finally up again to Refuge des Merveilles (be sure to book ahead as it's full in the summer time).
Day 2: Refuge des Merveilles - Refuge Valmasque
This run starts at the Refuge des Merveilles and passes south of Le Lac Long Superieur before you start the single, very manageable ascent to 2,565 meters (Baisse de Valmasque). On this part of the run, you'll be able to spot several petroglyphs on the rocks that you pass, and the notion of people having made these several thousand years ago is simply fantastic.
From the Baisse de Valmasque, you'll have beautiful views back along the trail, that you have just climbed, and forward toward the string of lakes that you'll be passing as you continue on. This part of the run follows along the GR52 Trail
From the Baisse, the trail winds down and the GR52 turns off to your left a little before you reach Lac du Basto. This is the first of a series of three very beautiful lakes (the following two being Le Lac Noir and Le Lac Vert) that you'll pass, before the trail ends at the Refuge Valmasque, where you should book your overnight stay in advance.
Day 3: Refuge Valmasque - Castérino
You start out from the wonderful Refuge Valmasque with its perfect view over Le Lac Vert. You start descending first over boulders and rocks but pretty soon you reach the first trees, and the rest of the run is primarily through forests.
Around one third of the way down, the singletrack trail widens and becomes a narrow gravel road, which finally gets paved as you get close to Castérino.
In Castérino, you can refresh yourself with a drink at one of the quaint small auberges and hotels.