Windows Server 2012 has a little bug out of the box. The .net 3.5 feature can’t be installed by using the “add remove features” and so do installs fail when trying to do this automatically.
In order to work around this problem, you can use DISM to install the .net feature.
Mount the Windows Server 2012 Media and run this command (change the commands drive letter if necessary)
dism /online /enable-feature /featurename:NetFX3 /all /Source:d:\sources\sxs /LimitAccess