Google Sketchup is free (there is another non-free version with more features) and is easy to use for 3D. I haven't used it for diagrams before but it could do it. You could fully model your designs and how they fit together and then put them in a virtual room and do walkarounds/fly-throughs.
It is easy to use compared with commercial CAD packages, I can give you some pointers with it if you download and try it.
The thing with the free version of Sketchup is that solids aren't really solid. You have to take care when joining parts together when removing lines (that would make sense if you have used it).
Of course, there is AutoCAD and SolidWorks if you like learning to use more complex software . Learning to use either of those is a bit of a project in itself though.