Read this full knowledgebase articles and know how to install .NET 3.5 on Windows 2012 R2.

To install .NET 3.5 you need to do the following

1. Mount the Windows 2012 R2 DVD, see: https://www.vpsblocks.com.au/support/Knowledgebase/Article/View/33/0/mount-a-dvd

2. Open a powershell window and type: Install-WindowsFeature -name Net-Framework-Core -source D:\sources\sxs\

Note: the above line assumes that the install DVD is mounted at D: