If during normal installation of .NET 3.5 an error is shown stating that the ISO or DVD is required follow the steps below to workaround this to get .NET 3.5 installed:
1.) Start the Local Group Policy Editor or Group Policy Management Console.
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) check box.
5.) Run the following command from a Windows Command Prompt as Administrator:
dism.exe /online /enable-feature /featurename:NetFX3 /all
.NET 3.5 is now installed.