1. Go to a command prompt and enter this:

dism /online /enable-feature /featurename:NetFX3 /all /Source:d:\sources\sxs /LimitAccess

Note: Source should be the Windows installation disc. In my case, this was located on D:

Bug when adding .net framework 3.5 in Server 2012

2. Go down to “Specify an alternate source path” and enter “d:\sources\sxs” as the path.

Specify alternate source path windows server 2012 .net framework 3.5

Specify alternate source path windows server 2012 .net framework 3.5

Now you should see this under your Features list:

.NET Framework 3.5 feature installed on Windows Server 2012