You can use the canvas "primitives" (Cube, Cylinder, Sphere, Cone, Wedge) at Rob's Imports to build 3D shapes, but they can be finicky to get right.
There are templates for non-standard canvas items in this thread.
Overall, it's about learning people's techniques and practice. To learn proper and interesting uses of canvas shapes, check out condos owned by people like Drrabbit, Computrix   or mystic . These three are very skilled builders and canvas users (Drrabbit especially), and you can learn a lot from just visiting their places.