I would highly recommend SketchUp for this kind of simple 3d 'sketching'. It's highly intuitive and well documented.

It was bought from Google a while back by a company called Trimble who made it kind of shit, but there are .exes for Google SketchUp 8 floating around. This lets you export in a wide variety of formats and runs beautifully in an XP VM if you are of the Linux persuasion