Bummer, I looked up the MSDN article about installing .NET 3.5.1 on Win8 and it turns out there is another way to add the feature so I gave it a shot:
No love there either. After a quick google search, DISM came to the rescue. Thanks to a couple guys on the TechNet forums, it’s a simple solution:
Make sure the Windows 8 disk is in the drive (if your drive is not the d:\ drive, change the source drive letter to match your drive)
dism.exe /online /enable-feature /featurename:NetFX3 /Source:d:\sources\sxs
The install will take a few minutes to run.
NOTE: This issue has also been identified on Windows Server 2012 while adding Roles and Features, as well as while using WDS.