3D Cartoon Drawing, or perspective drawing, requires not only artistic skills but also a good understanding of the technicalities that go with it. There are certain rules to be followed, which may be crucial to how your drawing comes out. Drawing in 3D form is a lot more difficult and potentially demanding and challenging. However, it isn't impossible to learn this and below is a few things you can do to begin drawing in 3D form.
1. Starting with the basics, pick a paper plate and a drawing pen. With it, start drawing a big circle in the middle of the plate. Once you have done this, look into what you have drawn and try to tilt the paper to level to your eyes and notice the dimension of the circle. The circle should have one part appearing as if it's closer than the other side.
You can also further illustrate this by drawing a square:
a. Put two linear dots apart in a piece of paper, and put two linear dots above and below the paper. You should have a cross of dots appearing.
b. Connect these together by forming a square. This is what is termed as foreshortening. It is when you're trying to draw an object in 3D, you are actually creating an illusion to trick the eye into seeing that a flat surface has some dimension in it.
2. Now that you have a basic idea of foreshortening, the next step is shading. This is where you take any flat drawing and shade some areas of it so that it would appear three-dimensional. For instance, with the circle on your paper:
a. Try shading the left side of it and see what happens. You shade your pencil from the line of the circle going to the right, presuming that the light is coming from the right side. Or you can do this the other way around if the light comes from the opposite side.
b. Be consistent with how you shade and blend this from dark to light. The blending is what gives your drawing a rounded appearance, thus it looks three-dimensional.
3. Finally, the last step is surfacing, or defining the surface of the drawing by giving it clear contour lines. As you sketch your squares and circles, as you shade them and make them appear with more dimensions, defining its outlines will also add character to the images. So, go ahead and make the lines darker. If your objects appear wrinkly, for example, you can emphasize this by darkening the contour lines of the wrinkles. If your object has curly hair, contour lines help define this, too.
These are pretty much what you need to do to begin drawing cartoons in 3D for. Eventually with some more work and mastering of the skill, you will be able to come out with drawings that are much more cleaner and finer.
To complement your work, there are a few books and courses in relation to drawing in three-dimension. If you are serious about learning this, it may be a good idea to take formal lessons.