Preheat the oven to 350 F.
Spray the three cake pans with baking spray.
In a large bowl, whisk together the flour, baking powder, and salt. Set aside.
Using a standing mixer, beat the butter and sugar together until creamy.
Beat in the eggs and vanilla extract until combined.
With the mixer on low speed, add the dry ingredients just until combined.
With the mixer still running on low, add the buttermilk, lemon zest, and lemon juice and mix just until combined.
Pour batter evenly into cake pans.
Bake for around 20-25 minutes, or until the cakes are baked through.
Remove the cakes from the oven then set to the side to cool.
Using a standing mixer, cream together all ingredients for the buttercream until stiff and smooth peaks form.
Using a cake slicer, slice a thin layer off the tops of the cakes to create a flat surface.
Place 1 cake layer on your serving plate.
Evenly cover the top with about 1 cup of frosting.
Top with the 2nd cake layer and evenly cover the top with about 1 cup of frosting.
Top with the third cake layer.
Spread the remaining frosting all over the top and sides.
Using a frosting leveler, smooth the frosting evenly and remove it to create a naked look.
Pipe larger dollops of frosting onto the top of the cake.
Place the sliced orange along the bottom of the cake.
Place the sliced lemons in between the larger dollops.
Let the cake sit for a minimum of 1 hour before serving.
Cut and enjoy!