Make Caramel: Combine sugar and water, heat until amber and pour into mold.
Whisk Custard: Mix eggs, condensed milk, evaporated milk, whole milk, zest, juice, vanilla, and salt. Strain.
Prepare Water Bath: Place mold in pan, fill halfway with hot water. Cover loosely with foil.
Bake: Bake at 350°F (175°C) for 50–60 min until center jiggles slightly.
Cool: Let flan rest 30 min in water bath, then chill 4 hours or overnight.
Unmold: Run knife around edges, flip onto plate, and serve with caramel sauce.