On windows 2012 R2 frequently have errors trying to install .Net FrameWork 3.5. The following command, run in elavated cmd box, installs it offline.
dism /online /enable-feature /featurename:netfx /all /source::\sources\sxs /LimitAccess
[Microsoft Technet Information] (https://support.microsoft.com/en-us/kb/2734782#/en-us/kb/2734782) .Net Framework 3.5 installation error: 0x800F0906, 0x800f081f, 0x800f0907