Preheat oven to 375°.
To prepare pâte á choux, lightly spoon the flour into dry measuring cups, and level with a knife. Combine water and next 3 ingredients (water through salt) in a medium saucepan; bring to a boil. Reduce heat to low, and add the flour, stirring well with a whisk until the mixture is smooth and pulls away from sides of pan. Remove from heat, and cool 5 minutes.
Add egg whites and egg yolk, 1 at a time, beating vigorously with a wooden spoon until smooth. Spoon mixture into a pastry bag fitted with a 1/2-inch round tip. Pipe 15 (1 1/2-inch round) mounds onto a baking sheet lined with parchment paper. Bake at 375° for 35 minutes or until golden. Remove pan from oven; reduce oven temperature to 300°. Cut a horizontal slit into one side of each cream puff. Return to oven; bake at 300° for 5 minutes. Cool on a wire rack.
To prepare coulis, combine the strawberries, blackberries, raspberries, and 3 tablespoons granulated sugar in a medium bowl; chill 30 minutes. Partially mash berries with a fork. Cut tops off cream puffs, and fill each cream puff with about 1 tablespoon yogurt. Sprinkle with powdered sugar. Serve with coulis.