This is due to having the machine that is controlled by WSUS policies and the .Net 3.5 not being approved in WSUS.
An easy way to install it manually is to install the Windows 8 Source media and run the following command from an administrative command prompt:
DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /LimitAccess /Source:d:sourcessxs
• Use /All to enable all parent features of the .NET Framework 3.5.
• Use /LimitAccess to prevent DISM from contacting Windows Update.
• Use /Source to specify the location of the files needed to restore the features
Make sure you replace /Source:d: with the drive letter or location of your install media. Also don’t forget, that natively in Windows 8 you can mount an ISO file by double clicking on it.