Essentially you need to provide Windows with the installation media to be able to install .Net Framework 3.5. That said, I found that if you try to provide the ‘Add Roles and Features Wizard’ with the installation source it doesn’t work. After some searching on the web I found I wasn’t the only person with this issue. So the solution is below using DISM.
From an elevated command prompt, run the following command:
dism /online /enable-feature /featurename:NetFX3 /all /source:d:\sources\sxs /limitaccess
This command uses d: as your source drive for the Windows Server 2012 media, replace that path with whatever you have. You should find that will add .Net Framework 3.5 for you and you’ll see the following success message:
If you would like to know a little more about the DISM tool, try here for a starting point.