Preheat oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan. Prepare the white cake mix according to package directions. Pour batter into the prepared pan and bake for 28-32 minutes, or until a toothpick inserted into the center comes out clean. Remove from oven and let cool in pan on a wire rack for 10 minutes.

While the cake is cooling, in a medium bowl, whisk together the boiling water and raspberry gelatin until completely dissolved. Using the handle end of a wooden spoon or a large straw, poke holes all over the warm cake, going about 3/4 of the way down. Slowly and evenly pour the liquid gelatin mixture over the entire cake, aiming for the holes. Refrigerate the cake, uncovered, for at least 1 hour to set.

In a medium bowl, combine the sliced strawberries, raspberries, and blueberries with the 2 tablespoons of granulated sugar. Gently toss and let sit at room temperature for 20-30 minutes. This draws out their natural juices, creating a flavorful syrup.

In a large, chilled bowl, combine the cold heavy whipping cream, 1/2 cup powdered sugar, and vanilla extract. Using a hand mixer or stand mixer fitted with a whisk attachment, beat on medium-high speed until stiff peaks form. Be careful not to over-whip into butter.

Remove the chilled cake from the refrigerator. Spread about two-thirds of the whipped cream evenly over the top of the cake. Spoon the macerated berries and their collected juices evenly over the whipped cream layer. Dollop or spread the remaining whipped cream over the berries. Garnish the top with the reserved 1/2 cup of fresh mixed berries.

Cover the cake loosely and refrigerate for at least 2 hours, or until ready to serve. This allows the flavors to meld and the cake to firm up for easier slicing. Dust with powdered sugar just before serving, if desired.