It is possible to install .NET Framework 3.5 not only through Windows features but also from the command line.
You can use Windows PowerShell by running the following command:
Install-WindowsFeature Net-Framework-Core -source \network\share\sxs
Alternatively, you can use the Command Prompt, so:
Click on START, type CMD, right-click on Command Prompt, and select Run as administrator.
Then type the following command:
DISM /Online /Enable-Feature /FeatureName:NetFx3 /All
If the operation fails, check if there are any pending in Windows Update.