It sounds like what you might be looking for is an online app called 'Sketchup' (sketchup.com) currently owned and maintained by Trimble. To access the free web-based version of Sketchup, select the "For Personal" tab, and create a free account (or use your gmail credentials) in order to use the free web-based version.
Sketchup is a very simple to use but powerful software that yields great results. They also have a large 3D library of objects that you can add to your 3D designs (saves you lots of time from having to construct many objects already created and ready for your use). I have used Sketchup to design theater sets. I have also used Sketchup to design a detailed layout of an addition to our home.
Check it out - you won't be disappointed!