Prepare a 9x9-inch glass baking pan by spraying lightly with cooking spray. With a paper towel, wipe spray until only a very light layer remains. This will help the gelatin release later.
Mix packages of blue jello with 1 envelope of gelatin. Add 2 cups of boiling water and stir until gelatin is completely dissolved. Pour into prepared pan and set in refrigerator about 40 minutes, until gelatin is set to the touch.
When gelatin is set to the touch, cut 1-inch strips with a hot knife. Dip the knife in hot water after each cut. You can use a large ruler to measure the strips or just eyeball it. Remove every other strip and discard them.
Mix packages of red gelatin with 1 packet of gelatin. Add 3 cups boiling water and stir until gelatin is completely dissolved. Add 2 tablespoons of red gelatin to each of the empty spaces (this layer will be lower than the blue layer). Return to fridge for 15-20 minutes, or until red gelatin is set. Keep the bowl with the remainder of red gelatin in a sink with hot water to keep it liquefied.
Combine 2 packets of gelatin with ¼ cup cold water. Let sit 2-3 minutes. Add 1 cup boiling water and stir to completely dissolve gelatin. In another bowl, mix ½ cup boiling water with the cup of sweetened condensed milk. Add milk mixture to gelatin mixture and stir to combine well.
Add 2 tbsp of white gelatin on top of each red gelatin to fill up to the same level as the blue gelatin, making sure it doesn't go over. The gelatin should all be on the same level now. Return to fridge for another 15-20 minutes or until set. Keep the bowl with the remainder of white gelatin in a sink with hot water to keep it liquefied.
Next, add 1 cup red gelatin over the entire pan. Return to refrigerator for another 15-20 minutes. Then add 1 cup white gelatin layer over the entire pan and cool for 15-20 minutes. Continue alternating red and white layers until all gelatin is used.
Refrigerate the completed layers for at least 2 hours or overnight to set. Checking the side of your glass pan to see where the blue gelatin is, cut ½-inch strips in the opposite direction. Then cut each strip into small flags.
Garnish your red, white, and blue jello flags or a serving plate as desired.
It is much easier to use a glass pan for this dessert as you need to know the directions of the stripes. The final cut will be in the opposite direction (which you won't be able to see if using a metal pan). However, if using a metal pan as I did, mark the side with an arrow in the direction of the stripes (not necessary if using a glass pan). *See photos in the post above.
Strawberry jello is lighter in color and maybe a better option than cherry (which I used).
Garnish a serving tray with red, white, and blue sprinkles.