Enable .NET Framework 3.5 by using the Add Roles and Features Wizard

You can use Server Manager to enable .NET Framework 3.5 for a local or remote installation of Windows Server 2012 R2 and later.

Requirements

Windows Server 2012 R2 or later
Installation media
Administrator user rights. The current user must be a member of the local Administrators group to add or remove Windows features.
Target Computers might need network access and rights to use either alternate sources or an Internet connection to use Windows Update.

In Server Manager, click Manage and then select Add Roles and Features to start the Add Roles and Features Wizard.
On the Select installation type screen, select Role-based or feature-based installation.
Select the target server.
On the Select features screen, check the box next to .Net Framework 3.5 Features.
On the Confirm installation selections screen, a warning will be displayed asking Do you need to specify an alternate source path?. If the target computer does not have access to Windows Update, click the Specify an alternate source path link to specify the path to the \sources\sxs folder on the installation media and then click OK. After you have specified the alternate source, or if the target computer has access to Windows Update, click the X next to the warning, and then click Install.

If you are using Server Manager to add a role or feature to a remote server, the remote server’s computer account (DOMAIN\ComputerName$) requires access to the alternate source file path because the deployment operation runs in the SYSTEM context on the target server.