With Windows 8 connected to the internet, you can:
1) From a elevated command line you can run:
DISM /Online /Enable-Feature /FeatureName:NetFx3 /All
Or
2) You can enable the .NET Framework 3.5 yourself through Control Panel. In Control Panel, choose Programs and Features, choose Turn Windows features on or off, and then select the Microsoft .NET Framework 3.5.1 check box.
From:
http://msdn.microsoft.com/en-us/library/hh506443%28v=VS.110%29.aspx
(There is a deployment guide at the bottom of the MS page that has other methods)
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