In a medium saucepan set over medium-low heat, combine the 3 cups frozen strawberries, 3 tablespoons water, ¼ cup granulated sugar, ¼ cup frozen lemonade concentrate, and zest and juice of 1 lemon. Stir well, cover, and cook until the strawberries are softened and the liquid is deep red, about 10 minutes.
Pour the liquid over a fine-mesh sieve into a bowl and add the 2 tablespoons strawberry gelatin powder into the liquid. Whisk well until the gelatin dissolves, and set aside, allowing the mixture to cool to room temperature, about 30-40 minutes.
While the filling cools, make the cream cheese layer. In the bowl of a stand mixer fitted with the paddle attachment, beat the 12 ounces cream cheese, ½ cup frozen lemonade concentrate, ¼ cup powdered sugar, and zest and juice of 1 lemon together on medium speed until smooth and creamy.
Set oven rack to middle position and preheat oven to 350°F (175°C). Spray a 9×13-inch baking pan with nonstick baking spray and set aside.
In the bowl of a stand mixer, combine the 15.25 ounces white cake mix, ½ cup frozen lemonade concentrate, ¾ cup water, ⅓ cup sour cream, 3 large eggs, and zest of 1 lemon. Beat together on medium speed for 2 minutes.
Pour the batter into the prepared pan. Bake according to cake package directions, or until a toothpick inserted in the middle comes out clean, about 25-30 minutes.
Transfer the cake to a wire rack to cool completely. When cooled, use a thin handle of a wooden spoon to poke about 35-40 holes all around the top of the cake.
When the strawberry gelatin mixture has cooled to room temperature, slowly pour (or spoon) it over and into the holes in the cooled cake.
Cover with plastic wrap (keeping it from touching the cake if possible) and refrigerate for 30 minutes before adding the Lemonade Cream Cheese Layer.
When the cake is ready, spread the Lemonade Cream Cheese mixture on top and smooth it out.
Cover with plastic wrap and refrigerate at least 4 hours or up to 2 days.
When ready to serve, decorate the top with 16 ounces Cool Whip, fresh strawberries, and colored sanding sugar, if desired.
