Berry Cake with Lemon Cream Mousse

batch lemon cream mousse (, can be made in advance)

white cake (, baked into three 8 or 9 inch round pans (from scratch, or a cake mix would work))

fresh raspberries (, about 12 ounces)

fresh blueberries  (, about 12 ounces)

fresh blackberries (, about 12 ounces)

fresh strawberries