Select all the shapes that make up the lateral side of your letters, click the Unite button from the Pathfinder panel (Window > Pathfinder), and turn the resulting shapes into a second compound path (Control-8). Focus on the Layers panel and open the existing layer. First, double-click on the name of your yellow compound path and rename it "front". Then focus on the compound path made in this step, rename it "3D" and make sure that it lies below your "front" compound path.