This is a well documented problem, with a well-documented fix.
The DISM command-line tool fixes things up nicely:
I encountered issues where the installation would complete to 66% -- every time -- and fail to complete. This was maddening, until I found the answer (no, I don't remember where.)
The problem was occurring because I had performed multiple updates on the server from Windows Update before installing the software. This caused the "image version" to be different -- this has to be the same for the .Net 3.5 installation to complete properly.
As such, whenever redoing my lab VM's, I always install the framework AS SOON AS POSSIBLE to ensure the tool will run properly.