Step 1: Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.
Step 2: Prepare cake mix according to package directions. Pour batter into the prepared pan.
Step 3: Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean. Let the cake cool in the pan for 10-15 minutes.
Step 4: While the cake is still warm, use the handle of a wooden spoon or a large fork to poke holes evenly over the entire surface of the cake, about 1 inch apart and going almost to the bottom.
Step 5: In three separate bowls, prepare each package of gelatin. Dissolve each package completely in 1 cup of boiling water. Stir for 2 minutes, then add 1/2 cup of cold water to each. Let the gelatin mixtures cool for 10-15 minutes until warm but not hot.
Step 6: Carefully spoon or pour the different colored gelatin mixtures into the poked holes, alternating colors randomly across the cake. Try to fill each hole generously.
Step 7: Refrigerate the cake for at least 3-4 hours, or preferably overnight, to allow the gelatin to set completely and infuse into the cake.
Step 8: Once the cake is completely chilled and the gelatin is firm, spread the thawed whipped topping evenly over the entire surface of the cake. Decorate with pastel sprinkles or candy, if desired. Slice and serve cold.