Bake cake (in a sheet cake pan) according to the package instructions. While cake bakes, boil pineapple with sugar until syrupy. When cake is baked, poke holes in top and pour the pineapple mixture on top; spread evenly. Cool completely. When cake is cool, mix the box of pudding with 3 cups milk and spread over cake. Next, mix the whipped topping with coconut and spread over cake. Refrigerate. (Cake is best if made 24 hours before serving).