Q: I'm trying to install .NET Framework 3.5 on my Windows Server 2012 (R2) servers, but it won't install. Why not?
A: .NET Framework 3.5 isn't part of the local cache of binaries on a deployed server. Therefore, it actually has to be downloaded when it's enabled. It can be downloaded through Windows Update (by default), or you can download it to a specific machine or share. If a server is configured to use WSUS, then it will try to use WSUS but will fail. There are several workarounds.
The first workaround is to set a registry key on the server that tells it to not use WSUS for downloading .NET 3.5 and instead use Windows Update.
[HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\Servicing] "RepairContentServerSource"=DWORD(2)
Another option is to use the Deployment Image Servicing and Management (DISM) tool and just point to the command at a folder containing .NET Framework 3.5. This could be the original installation media sources\sxs folder or a folder you copied it to.
Dism.exe /online /enable-feature /featurename:NetFX3 /All /Source:d:\sources\sxs /LimitAccess
