The hike around Tre Cime starts (and ends) at Rifugio Auronzo, one of countless mountain huts in the Italian Dolomites.
Cortina d'Ampezzo is over the low pass beyond and to the right of Lake Misurina. It's about 20km from Cortina to Rifugio Auronzo.
The first section of the trail is one of the busiest in the Dolomites.