Preheat oven to 325 degrees F (165 degrees C). Grease 2 9-inch round pans.
Combine dry cake mix and gelatin powder in a large bowl; add 1 package raspberries with juice, eggs, vegetable oil, and hot water. Beat mixture thoroughly; divide batter between prepared round pans.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 28 to 35 minutes. Cool in the pans for 10 minutes before removing to cool completely on a wire rack.
Fold 1 package thawed frozen raspberries through whipped topping in a bowl; spread about half the frosting evenly over 1 cake, place other cake atop the frosted cake, and frost with remaining frosting. Refrigerate at least 2 hours.
Top frosted cake with halved strawberries along outer edge and with raspberries in a circular pattern in the middle. Place blackberry in center of cake.
This recipe can also be made in a larger rectangular dish and just frosted on the top, making it great for potlucks, cookouts, camping and any other group gathering where a standard cake doesn't work out so well. Just up oven to 350 degrees F and bake for 30-35 minutes instead.
Let cake cool completely before frosting. I put the cake pans in the freezer for half an hour to speed the cooling process. If you do this, set a timer so you don't freeze the cakes solid.