A: The .NET Framework 3.5 (and 3.0 and 2.0) is actually included on the Windows 8 and Windows 2012 source media in the sources\sxs folder. This can be used as an alternate location for the feature install routine to pull the required binaries.
For example, you could use the following Windows PowerShell command:
Install-WindowsFeature NET-Framework-Core -Source D:\sources\sxs
The above is assuming the CD-ROM is in the D drive or the sources\sxs contend could be copied to a network share.
When using the Add Roles and Features option in Server Manager on the Confirm installation selections page, the option Specify an alternate source path is available, which could also be used to select the sources\sxs folder (see the screen shot below).
Additional information can be found at this Microsoft TechNet article, and a good whitepaper is available, titled "Microsoft .NET Framework 3.5 Deployment Considerations."