Pierce the cake with a fork to help the syrup soak in before you cover the cake with meringue. The name of the cake refers to the three types of milk used in the recipe.
Preheat the oven to 375°.
To prepare cake, coat a 13 x 9-inch baking pan with cooking spray; dust with 1 tablespoon flour. Set aside.
Place egg yolks, 2/3 cup sugar, and 2 teaspoons vanilla in a large bowl; beat with a mixer at high speed until mixture is thick and pale, about 2 minutes.
Lightly spoon 2 cups flour into dry measuring cups; level with a knife. Combine flour, baking powder, and 1/4 teaspoon salt; stir well with a whisk. Add flour mixture and 1/2 cup fat-free milk alternately to egg yolk mixture, beginning and ending with flour mixture.
Beat 6 egg whites in a large bowl with a mixer at high speed until foamy using clean, dry beaters. Add 3 tablespoons sugar, 1 tablespoon at a time, beating until stiff peaks form (about 2 minutes). Gently fold egg white mixture into flour mixture. Spoon batter into prepared baking pan. Bake at 375° for 20 minutes or until wooden pick inserted in center comes out clean. Cool in pan 10 minutes on a wire rack. Turn cake out onto a platter with sides. Pierce entire top of cake with a fork.
To prepare sauce, combine condensed milk, evaporated milk, 1/2 cup fat-free milk, and rum; stir with a whisk. Slowly pour milk mixture evenly over cake. Cover with plastic wrap, and refrigerate at least 3 hours or overnight.
To prepare meringue, beat 4 egg whites, cream of tartar, and 1/8 teaspoon salt in a large bowl with a mixer at high speed until foamy using clean, dry beaters. Combine 1 cup sugar and 1/2 cup water in a saucepan; bring to a boil. Cook, without stirring, until candy thermometer registers 250°. Pour hot sugar syrup in a thin stream over egg whites, beating at high speed until stiff peaks form (about 3 minutes). Stir in 1 teaspoon vanilla. Spread meringue evenly over top and sides of cake. Garnish with nutmeg, if desired.