Take your 3 cookie sheets and line them with parchment paper. Preheat your oven to 300F.
Once your popcorn is cooked, take and spread 3 cups of popcorn out in an even layer on each cookie sheet. Take and sit to the side until ready to use again.
Using a medium saucepan, place it over medium heat and start combining the 3 Tablespoons butter, the cup of sugar, and also adding in the 1 Tablespoon white corn syrup and one color of the Jell-O powder. (your choice) Bring to a boil, adding water or food dye, as needed. (Your only going to add in the water if the mixture starts to get too thick.)
Once everything is added, boil for 3-4 minutes. Once done, take and pour the candy mixture over one cookie sheet of popcorn.
Place that cookie sheet in the oven and bake the popcorn for 10 minutes, making certain to stir about halfway through.
Important! Let cool before handling. Once you're done with one, go ahead and repeat with each color of the Jell-O.