The .NET Framework 3.5 has been removed and cannot be installed without media.

PS C:\> Get-WindowsFeature *Framework*

Display Name                                            Name                       Install State
------------                                            ----                       -------------
    [ ] .NET Framework 4.5                              AS-NET-Framework               Available
[ ] .NET Framework 3.5 Features                         NET-Framework-Features         Available
    [ ] .NET Framework 3.5 (includes .NET 2.0 and 3.0)  NET-Framework-Core               Removed
[X] .NET Framework 4.5 Features                         NET-Framework-45-Fea...        Installed
    [X] .NET Framework 4.5                              NET-Framework-45-Core          Installed
    [ ] ASP.NET 4.5                                     NET-Framework-45-ASPNET        Available
[ ] Windows Biometric Framework                         Biometric-Framework            Available

Either:

- Supply installation media (sources\sxs)
- Set the Specify settings for optional component installation and component repair Group Policy
- Set Specify settings for optional component installation and component repair in the registry

REG ADD "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\policies\Servicing" /V LocalSourcePath /D "" /T REG_EXPAND_SZ /F
REG ADD "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\policies\Servicing" /V RepairContentServerSource /D 2 /T REG_DWORD /F

Alternatively, pre install the feature from the source media to allow normal feature installation via Server Manager or PowerShell:

dism.exe /online /enable-feature /featurename:NetFX3 /All /Source:"\\server\share$\media\sources\sxs" /LimitAccess

.NET Framework 3.5 installation error: 0x800F0906, 0x800F081F, 0x800F0907
https://support.microsoft.com/kb/2734782

You cannot enable the Microsoft .NET Framework 3.5 feature on Windows Server 2012 R2, Windows Server 2012, Windows 8.1, or Windows 8 after you install security update 2966827 or 2966828
https://support.microsoft.com/kb/3002547

http://woshub.com/how-to-install-net-framework-3-5-in-windows-server-2012-r2/