Coat an American flag mold with cooking spray and place on a baking sheet.
Combine 1 1/2 cups boiling water and red gelatin mix in a medium-sized bowl. Stir until dissolved, at least 2 minutes. Add 1 1/2 cups cold water and stir. Pour red gelatin into the prepared mold and refrigerate until set, but not yet firm, about 45 minutes; gelatin should stick to finger when touched and should mound.
Wait 20 minutes after you put the red gelatin into the refrigerator. Combine 1 cup boiling water and lemon gelatin in a bowl. Refrigerate until slightly thickened, about 25 minutes.
Remove lemon gelatin from the refrigerator. Whisk in thawed whipped topping until combined and spread mixture over red gelatin in the mold. Return to the refrigerator until set but not firm, about 10 minutes.
Combine 1 1/2 cups boiling water and blue gelatin in a medium-sized bowl. Stir until dissolved, at least 2 minutes. Stir in 1 1/2 cups cold water. Pour blue gelatin into the mold. Refrigerate until firm, at least 4 hours. Unmold onto a serving tray.
