In sculptor, you could select a 3 column layout as a starting point, then adjust each column accordingly to the size you want. After that, add a custom div above the contentWrapper. This would give you a larger area above the 3 columns (or boxes if you make then equal width) and should give the basic effect like on the WA home page. The box tab is really powerful in allowing you to add and position custom divs to achieve a more complex layout like you want.