In a large bowl, dissolve the red gelatin in 2 cups of boiling water. Stir until completely dissolved. Add 1 cup of cold water and mix well. Pour the mixture into a 9x13-inch baking dish and refrigerate until set, about 2 hours.

Once the red layer is set, prepare the white layer. In a saucepan, sprinkle the unflavored gelatin over 1 cup of cold milk and let it sit for 5 minutes. Heat the remaining 1 cup of milk in the saucepan over medium heat until warm, but not boiling. Stir in the sugar and vanilla extract until dissolved. Remove from heat and add the softened gelatin mixture, stirring until fully dissolved. Allow to cool slightly before pouring over the red layer. Refrigerate until set, about 2 hours.

After the white layer is set, prepare the blue layer by dissolving the blue gelatin in 2 cups of boiling water. Stir until completely dissolved, then add 1 cup of cold water and mix well. Pour the blue mixture over the white layer and refrigerate until fully set, about 2 hours.

Once all layers are set, cut the jello into squares or use cookie cutters to create fun shapes. Serve with whipped topping on the side.