There are a few freeware programs out there that will do what you want that are used by many DIY Designers / Builders. While I have not used them myself, the feedback on them seem to be good. Give me some time and I will see if I can find the links for you.
"All things are possible, once you overcome your fears."
I use DesignCAD for doing wiring diagrams. It does 3D and may work for you. It is not costly either. Post your question here: DesignCAD User Forum - Index. People who use it for the purpose you need can tell you precisely how well it would do.
This looks interesting. I have not looked at it either, but notice that it has a big brother, Sketchup Pro, that has a hefty price tag. Maybe someone can tell us if the free one has enough capability, or if is there just to lead you into the $795 version.