You can configure server to use Windows Update as a source for for optional components installation. And then install .NET 3.5 through the Add Roles and Features Wizard.
This worked for me. Windows has to download the .NET 3.5 installation files, but the server is configured not to use Windows Update (common for managed servers), but WSUS.
1. Start the Local Group Policy Editor or Group Policy Management Console (WIN+R and type gpedit.msc).
2. Expand Computer Configuration, expand Administrative Templates, and then select System.
3. Open the Specify settings for optional component installation and component repair Group Policy setting, and then select Enabled.
4. Select the Contact Windows Update directly to download repair content instead of Windows Server Update Services (WSUS) checkbox.
Now start installing .NET Framework 3.5 through the Add Roles and Features Wizard and get it installed successfully.