Heat the oven to 350°F (175°C). Grease three 8-inch round cake pans and line the bottoms with parchment.

Whisk the flour, baking powder, and salt together in a medium bowl.

Beat the softened butter and granulated sugar for 3–5 minutes, until very light and fluffy. Add the eggs one at a time, then beat in the vanilla.

Add the dry ingredients in three additions, alternating with the buttermilk. Mix only until the batter is combined.

Divide the batter evenly among the prepared pans. Bake for 25–30 minutes, until the centers spring back and a toothpick inserted in the middle comes out clean.

Cool the cakes in their pans for about 10 minutes, then turn them onto wire racks and cool completely. Level the tops if necessary.

Combine the strawberries, blueberries, raspberries, sugar, and lemon juice. Let the berries stand for 15–20 minutes, then drain off excess liquid.

Beat the cream cheese, mascarpone, powdered sugar, vanilla, and salt until smooth. In a separate bowl, whip the cold heavy cream to medium-stiff peaks, then fold it into the cheese mixture in two or three additions.

Place one cake layer on a serving plate, spread it with frosting, and add a thin layer of berries. Repeat with the second layer, then add the final cake layer and frost the top and sides.

Decorate with additional whole berries and refrigerate the assembled cake for at least 1 hour before slicing.