Overview
When installing .NET 3.5 on server 2012 often it will error out.
To make this work you have to choose to install from an "alternate source path"
Then you have to point to the 2012 install media, for example:
D:\Sources\SxS
.NET 3.5 should install if the path is correct.
Alternate Approach
I've come across situations where the above method didn't work and found another solution.
It involves using the local group policy editor and specifying the alternate source path, which allows you to install normally without specifying the path in server manager.
Computer Configuration\Policies\Administrative Templates\System\Specify settings for optional component installation and component repair
Specify an alternate source path or check the box next to "Contact Windows Update directly ...."
Or you can do this globally via AD and Group Policy See this technet article: https://technet.microsoft.com/en-us/library/dn482065.aspx
Power Shell Method
Yet another way to install .NET 3.5
Install-WindowsFeature Net-Framework-Core -source D:\sources\sxs
Where D: is where your windows 2012 media is mounted.
Reference
https://technet.microsoft.com/en-us/library/dn482071.aspx http://serverfault.com/questions/463604/unable-to-install-net-3-5-on-windows-server-2012