Butter and flour three 8-inch round cake pans.
Combine cake mix, eggs, 7Up or Sprite, and butter. Mix according to the times listed on the cake mix box.
Divide batter evenly among the three pans. The batter will be thick and may need to be gently spread to the edges with a spatula.
Bake at the temperature listed on the cake mix box for about 20 minutes, or until a toothpick inserted in the center comes out clean.
Cool cakes in pans for 5 to 10 minutes. Remove from pans and cool until just warm to the touch.
If cakes have domed tops, trim them flat and brush away loose crumbs.
Stir the sweetened condensed milk until smooth.
Brush or spoon the condensed milk generously over the tops of each cake layer, allowing it to soak in.