.NET 3.5 and previous verions do not come standard on Windows Server 2012 R2 and tripped me up the first time I needed to install it for a software installation. Its easy enough to get through but I thought this would be a great quick write up in case anyone ran into issues or were curious of the process. You will need your 2012 R2 installation disk for this process.
Step one is opening the server manager
Next click on Add roles and features
Click Next
Make sure Role Based or Feature Based installation is selected and click Next
Make sure Select a server from the server pool is checked and your server is highlighted and click next (may be different for more complex installations)
Click Next
Select .Net Framework 3.5 Features and click Next
On the Confirmation page select Specify an alternate source path
On the Source Path window that pops up note the part that talks about where these source files would be found. We will need to pull them from the 2012 R2 installation media. Place it into your computer (or mount it to the server if in a virtual environment). We will go to the noted path and copy the address into the field next to the word Path:.
Open Windows Explorer and navigate to "This PC", right click on the media drive and select Open
Open the Sources folder
Open the "sxs" folder
Move your cursor to the address bar and highlight the entire path, for me it is D:\sources\sxs
Copy this to your clipboard
Paste the path into the Path: box in the Source Path window and click OK
Click Install and you are finished! If you have any issues just post up and I will try to help.