Prepare cake as directed on box. Spray 9x13 baking dish with oil. Bake at 350 until center of cake is done. Remove cake and immediately poke holes down into cake about every 3 inches (I use the round end of a wooden spoon). Then pour can of sweetened condensed milk over cake and into holes. Repeat with hot fudge and caramel topping (I remove the lids on the toppings and microwave a bit to warm them up for easy pouring). Make sure the milk and toppings get down sides of cake and into all the holes. Allow cake to cool.
When cake is completely cool (an yes, it doesn't look pretty right now, but wait), top with the cool whip, spreading to make it pretty. Sprinkle the top with the crushed Heath bars and refrigerate until serving time. This cake will soon become everyone's favorite!