Q: I'm trying to install .NET Framework 3.5 on my(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.
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.