These are just a bunch of random doodles and sketches I drew in school. I know it's not the best art and i have to work on my 3 dementionalism but hey whatever.
I think you did a great job on a lot of these.
and for making them look more three dimensional. you would do that with your shade and highlights. so you would want to figure out what direction you want your light source to come from, and you want your darker colors on the back side and your lighter colors towards the light source.