Manually its already possible to draw simple 3D draws like the above example (its basically just duplicate and move).
I have now read this manual
on how to make 3D boxes, but how do I apply transparency to e.g. a red box?
I mean; If I just have a red box, can Inkscape then figure out what the colour of the others sides should be, if I want transparency?
There is an extension (dropshadow),which can be used to program a new extension to build basic 3D primitives. I think it would be more cool, to have a cube/cone/etc tool like the existing rectangle tool. So I would prefer the interactive way (not via extensions).
Cool! I got to try dropshadow =)