Preheat the oven to 350°F (175°C). Grease three 8-inch round cake pans and line the bottoms with parchment paper.
Whisk together the flour, baking powder, and salt in a medium bowl.
In a large bowl, beat the softened butter and sugar until pale, light, and fluffy, about 2 to 3 minutes.
Add the eggs one at a time, mixing well after each addition. Mix in the vanilla extract.
Add the dry ingredients and milk alternately, beginning and ending with the dry ingredients. Mix just until the batter is smooth. Do not overmix.
Divide the batter evenly among three bowls. Leave one portion plain. Color one portion red and another blue using gel food coloring.
Pour each batter into a prepared cake pan and spread evenly.
Bake for 20 to 25 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Cool the cakes in their pans for about 10 minutes, then transfer them to a cooling rack. Allow them to cool completely.
To make the frosting, beat the softened butter until creamy. Gradually add the powdered sugar, followed by vanilla, salt, and enough cream or milk to create a smooth, spreadable consistency.
If necessary, level the tops of the cooled cake layers with a serrated knife.
Place the red cake layer on a serving plate and spread frosting over the top. Add the white layer, followed by another layer of frosting. Finish with the blue cake layer.
Spread a thin crumb coat over the entire cake and refrigerate for 15 to 20 minutes
Apply the remaining frosting and smooth the top and sides with an offset spatula.
Arrange the fresh strawberries and blueberries over the top of the cake.
Chill the finished cake for 20 to 30 minutes before slicing and serving