You may encounter .NET Framework 3.5 error while installation of feature on windows server 2012 R2 and .net frameworks features cannot be installed. As shown in below screenshot in Add Roles and Feature wizard.
Installation of one or more roles, role services, or features failed.
The source files could not be found. Try installing the roles, role services, or features again in a new Add Roles and Features Wizard session, and on the Confirmation page of the wizard, click "Specify an alternate source path" to specify a valid location of the source files that are required for the installation. The location must be accessible by the computer account of the destination server.
.NET Frameworks 3.5 Features
.NET Framework 3.5 (includes .NET 2.0 and 3.0)
To resolved this issue you must have windows 2012 R2 ISO file or DVD, (If you have direct internet connection these steps are not necessary and it will download required files for installation) Once you select .Net frameworks 3.5 feature in server manager add roles and features wizard, click Next.
Here you must mount OS ISO file or insert DVD in the rom. Click on specify an alternate source path, Provide the OS files location, in my case D drive is my CDROM, so my location will be D:\Sources\sxs, in you case replace D with CDROM Drive or file path.
There is another procedure if you want to install this feature using Powershell use command
Install-WindowsFeature 'NET-Framework-Core' -Source D:\sources\sxs
If everything is good you get success true and Exit code Success on the console output.
Another commandline can be use for same task.
Dism.exe /online /enable-feature /featurename:NetFX3 /All /Source:d:\sources\sxs /LimitAccess
If you don't have ISO or DVD computer must be configured to take updates from Microsoft directly, and run above procedures without specifying source, It will install .net feature on windows 2012 r2 without ISO or DVD.
This blog is also application for Windows 8.x and Windows 10, for more on this check out https://support.microsoft.com/en-in/kb/2734782.