Step 1
Preheat oven to 350 degrees F (175 degrees C). Spray a 9×13-inch baking dish with cooking spray and line with parchment paper.

Step 2
Mix cake mix, 1 cup water, eggs, and vegetable oil in a stand mixer on low speed for 30 seconds. Increase speed to medium and mix for 2 minutes more. Pour batter into the prepared baking dish.

Step 3
Bake in the preheated oven until cake is golden brown, about 25 minutes. Cool cake at room temperature for 10 minutes. Keep oven on.

Step 4
Line a baking sheet with parchment paper.

Step 5
Mix graham cracker crumbs and melted butter together in a bowl; spread onto the prepared baking sheet.

Step 6
Bake graham cracker crumb mixture for 10 minutes. Cool to room temperature and store in an airtight container.

Step 7
Mix strawberries, blackberries, raspberries, sugar, 1/3 cup water, and 2 tablespoons lemon juice together in a saucepan; bring to a boil. Reduce heat and simmer, stirring occasionally, until fruit is soft and easily mashed, 7 to 8 minutes. Mash berries together with a fork and cool for 5 minutes.

Step 8
Poke 5 rows of 6 holes in the cake using the handle of a wooden spoon, creating 30 holes. Drizzle 1/3 cup sweetened condensed milk over the entire cake. Pour berry mixture over cake and spread using a spatula, ensuring berry mixture goes into the holes. Cover cake and refrigerate at least 1 hour.

Step 9
Place cream cheese in the bowl of a stand mixer fitted with a whisk attachment; mix on high until very smooth, at least 3 minutes. Add the remaining sweetened condensed milk, 1/4 cup lemon juice, and sea salt; mix for 1 minute. Spread cream cheese mixture over the cooled cake. Cover and chill cake, 3 hours to overnight.

Step 10
Slice into 12 large pieces and top each with graham cracker crumb mixture.