Illustrating objects to give it a 3D look requires a few transforms, unions, differences & most importantly the gradient. Here is a simple circle you can draw. The technique used here applies only for basic shapes and that too for particular orientations.
Only 3 objects is all you need to get this cool 3D looking ring.
here are the steps:
step1: Draw an ellipse (let the opacity be around 75% for our visualization)
Duplicate it and scale it to a smaller size holding shift & control.
step2: Duplicate these ellipses and move them out.
Duplicate them once more and shift them vertically up by few pixels(as in image)
step3: Select the two ellipses, duplicate them and move them out.
then do path-->difference
We have our object 1.
step4: Select the two smaller ellipses from the object in step 2, duplicate them and move them out.
Now do path-->difference. (check out which object should be above which one )
We have our object 2.
step5: Select the two larger ellipses from the object in step 2, duplicate them and move them out.
We need to draw a rectangle. Its width same as the width of the ellipse
and height equal to the vertical distance between the ellipses.
Select the lower ellipse and the rectangle and do path-->union. Move this object to bottom.
Select both these objects and apply path-->difference.
We have your object 3.
step6: Now what you need to do is assemble these objects.
Zoom in and arrange them.
step7: Next we need to color them.
for object 1, a plain fill is OK.
for object 2, we need a linear gradient with variation 'dark to light to dark'
for object 3, we need a linear gradient with variation 'light to dark to light'
make the opacity to 100%
PS: when aliging objects, there may be some gaps/white spaces between them.
to hide it, give the object a stroke with the same color/gradient of the fill.
You can do similarly for triangle and square shapes. A representation is here in the PNG & SVG files.
Grouping these objects, you can do cool 3D illustrations like this and many more....
For complex objects, and even for simple objects with different orientations, above technique is not suitable.Will try to cover that in next post.
Happy Inkscaping :)