There are some legacy applications that require .NET Framework 3.5 or older on Windows 8.1. This is a feature that you should be able to add via Windows Feature or download from Windows Updates, but that didn't work for me.
Windows 8.1 Enterprise failed to .Net 3.5 using either method, so I went back to the source. Using the Win8.1 Ent media on USB flash drive, you can add the feature by specifying the source folder.
From and Administrative command prompt:
# Set the drive letter for your flash drive, in my case e:
DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /LimitAccess /Source:e:\sources\sxs
I created a batch script with this command and added to the root of my Win8.1 flash drive to save time in the future.