In a large bowl, whip heavy cream with an electric hand mixer until stiff peaks form. Beat in condensed milk until well blended.

Spread 1 cup of the cream mixture into the bottom of a 9x13-inch baking dish. Sprinkle half of the red gelatin over the top. Add another thin layer of cream mixture. Sprinkle with half of the yellow gelatin. Add another thin layer of cream mixture. Sprinkle with half of the blue gelatin. Repeat layers, ending with the cream mixture, until all of the cream mixture has been used - you may not use all of the gelatin mixes. Save remaining gelatin for another use (like another batch of ice cream!) or discard any unused portion.

Place in freezer for at least 8 hours or overnight.