Preheat oven to 250°.
Cover a baking sheet with parchment paper. Draw 8 (3-inch) circles on paper. Turn the paper over; secure with masking tape. Beat the egg whites and cream of tartar with a mixer at high speed until soft peaks form. Gradually add 3/4 cup sugar, 1 tablespoon at a time, beating until stiff peaks form (do not underbeat). Divide egg white mixture evenly among 8 drawn circles on baking sheet. Shape the meringues into nests with 1-inch sides using the back of a spoon. Bake at 250° for 1 hour. Turn oven off; cool meringues in closed oven for at least 2 hours or until completely dry. Carefully remove the meringues from paper.
Spoon yogurt onto several layers of heavy-duty paper towels, and spread to 1/2-inch thickness. Cover with additional paper towels; let stand for 5 minutes. Scrape into a bowl using a rubber spatula. Combine 1/2 cup sugar, drained yogurt, and crème fraîche. Spoon the yogurt mixture evenly into the meringues. Top each serving with 2 tablespoons each of strawberries, blackberries, and blueberries.