01 - Preheat oven to 350°F. Position the rack in the center of the oven.
02 - In a heavy saucepan, combine granulated sugar and water. Cook over medium heat without stirring until the sugar dissolves and turns a golden amber color, approximately 7 to 9 minutes. Immediately pour the hot caramel into a 9-inch round baking dish or flan mold, tilting to evenly coat the bottom. Set aside to harden.
03 - In a large mixing bowl, whisk together the eggs, egg yolks, and salt until well combined. Add the sweetened condensed milk, evaporated milk, whole milk, vanilla extract, and orange zest if using. Whisk until the mixture is smooth and homogeneous.
04 - Pour the custard mixture through a fine-mesh sieve into the caramel-lined baking dish to ensure a silky, lump-free texture.
05 - Place the flan dish inside a larger roasting pan. Carefully fill the roasting pan with hot water until it reaches halfway up the sides of the flan dish, creating a gentle water bath for even cooking.
06 - Bake for 60 minutes, or until the center barely jiggles when gently tapped and a knife inserted near the edge comes out clean.
07 - Remove the flan dish from the water bath and allow it to cool to room temperature. Then refrigerate for at least 3 hours or until thoroughly chilled and set.
08 - Run a thin knife around the edge of the flan to loosen it. Place a large serving plate upside down over the baking dish, then invert quickly to release the flan with the caramel layer on top. Slice into wedges and serve chilled.