Preheat oven to 350°F (175°C). Place a 9-inch round cake pan or flan mold inside a large roasting pan. Boil water in a kettle for the water bath.

Make the caramel by heating 1 cup sugar in a medium saucepan over medium heat. Stir constantly with a wooden spoon as it melts and turns amber, about 6-8 minutes. Watch closely to prevent burning.

Immediately pour the caramel into the bottom of the 9-inch pan, tilting to coat the bottom evenly. Work quickly as it hardens fast. Set aside to cool.

In a large bowl, whisk the eggs until well beaten. Add the sweetened condensed milk, evaporated milk, whole milk, vanilla extract, and salt. Whisk until completely smooth with no streaks.

Pour the custard mixture through a fine-mesh strainer directly into the caramel-lined pan. This removes any lumps or foam for a silky texture.

Place the roasting pan with the flan pan inside on the oven rack. Carefully pour hot water into the roasting pan until it reaches halfway up the sides of the flan pan.

Bake for 50-60 minutes, until the edges are set but the center still jiggles slightly when gently shaken. A knife inserted halfway between center and edge should come out mostly clean.

Remove from the water bath and let cool to room temperature on a wire rack, about 1 hour. Cover tightly with plastic wrap and refrigerate for at least 4 hours or overnight.

To unmold, run a thin knife around the edge of the pan. Place a large rimmed serving plate upside-down over the pan. Flip both together in one smooth motion. The flan will release with the caramel sauce flowing over the top.