I made a quick edit of the third peice

some observations:
-you have alot of banding
-you tend to shade things equally everywhere, so theres not much indication of the direction of the light source.
-you tend to outline shapes with darker shades inside the outlines. This adds to the banding, and confuses the shading, making it look flat.
-the outlines are inconsistent. Theyre black in some places, but not in other places.