Place cherry gelatin mix into a heatproof bowl, stir in 1 packet of unflavored gelatin, and mix with 2 cups of boiling water, stirring until the gelatin has dissolved and the mixture is clear, about 2 minutes. Pour into the bottom of a 9x13 rectangular glass baking dish. Place in refrigerator until completely cold and set, about 1 hour.
Sprinkle 4 packets of unflavored gelatin onto 1 cup of boiling water in a heatproof bowl, allow to stand for about 3 minutes, and stir until dissolved; mix in the sweetened condensed milk. Allow to cool but not thicken, about 10 minutes; gently pour into a layer on top of the cherry layer. Return to refrigerator and allow to chill completely, about 1 more hour.
Place blue raspberry gelatin mix and 1 packet of unflavored gelatin into a heatproof bowl, and mix with 2 cups of boiling water, stirring until the blue gelatin is clear, about 2 minutes. Allow to stand and cool but not thicken, about 10 minutes; gently pour onto the white layer. Chill in refrigerator until the dessert is firm and cold, about 1 hour. To serve, cut into squares.
