Preheat your oven to 350°F (175°C) and prepare three 8-inch round cake pans by lining the bottoms with parchment paper and lightly greasing the sides.

In a medium bowl, whisk together the flour, baking powder, and salt; set this dry mixture aside.

In a large mixing bowl, beat the softened butter, vegetable oil, sugar, and vanilla extract together for about 2 minutes on medium-high speed until the mixture becomes pale and fluffy. This step is crucial, as proper creaming enhances both texture and rise.

Add the eggs, one at a time, ensuring each is fully incorporated before adding the next. Scrape down the sides of the bowl occasionally to ensure an even mixture.

Gradually add half of the dry ingredients to the creamed butter mixture and mix until just combined.

Slowly pour in the milk while mixing on low speed until well blended. Follow by adding the remaining dry ingredients, mixing just until the batter is smooth. Avoid overmixing, as it can result in a dense cake.

Evenly divide the batter between the three prepared cake pans and bake for 22-25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.

Allow the cakes to cool in the pans for 2-3 minutes, then carefully transfer them to wire racks to cool completely.

In a medium saucepan, combine the sugar, cornstarch, water, and lemon juice. Heat the mixture over medium heat, stirring frequently, until the sugar dissolves and the mixture becomes glossy.

Add the chopped strawberries, blueberries, and raspberries to the saucepan, stirring to coat the berries evenly in the syrup.

Continue cooking the berries for about 3-4 minutes, gently mashing them with a spatula as they soften. The juices will thicken slightly as the mixture simmers.

Once the filling has thickened, remove it from the heat and transfer it to a bowl. Refrigerate until completely cooled.

In a large mixing bowl, combine the mascarpone and cream cheese. Using a whisk attachment, beat on low speed until smooth and well-blended. Avoid overmixing, as mascarpone can quickly become runny.

Gradually pour in the heavy whipping cream a few tablespoons at a time, whisking gently to incorporate it with the cheese mixture. This step ensures a smooth and lump-free cream.

Once all the cream has been added, increase the speed to high and whip the mixture until it thickens to stiff peaks.

Finally, add the powdered sugar and vanilla extract, whipping just until the cream is fully combined and holds its shape.

Level the tops of your cooled cakes with a serrated knife to ensure a flat surface. Place the first cake layer on a serving plate or cake board.

Pipe a thin "dam" of Chantilly cream around the outer edge of the cake. Fill the center with half of the cooled berry filling, spreading it evenly.

Pipe a layer of Chantilly cream over the berry filling and smooth it with a spatula.

Place the second cake layer on top and repeat the process with the remaining berry filling and another layer of Chantilly cream.

Add the final cake layer and frost the entire cake with the remaining Chantilly cream. Use a spatula to create a smooth finish and pipe decorative swirls on top if desired.

Garnish the cake generously with fresh strawberries, blueberries, and raspberries for a vibrant, elegant finish.

Refrigerate the cake until ready to serve. For the best texture and freshness, store the cake in an airtight container in the refrigerator and consume within 2-3 days.