Windows Server 2012 R2 includes the .NET Framework 4.5.1 as an OS component, and it is installed by default. It also includes the .NET Framework 3.5 SP1 as an OS component that is not installed by default.
If you run Setup(Microsoft SQL Server 2014 R2) and you do not have .NET 3.5 SP1, SQL Server Setup requires you to download and install .NET 3.5 SP1 before you continue with the SQL Server installation.
If you are trying to install .NET Frame 3.5 from the Server Manager GUI, you will see the below alert when installing the feature.
"Do you want to specify an alternate source path? One or more installation selections are missing source files on the destinatino server.…”
If your computer does not have access to windows update, then specify an alternate source path by clicking on "Specify an alternate source path" and enter the path like "C:\sources\sxs"
After you have specified the alternate source, close the warning (by clicking the x), then click Install.