I can’t tell how many times I have been burned when trying to simply enable .NET Framework 3.5.1 (and all the sub-goodies that so many programs need).  Lately, my struggle has been with Windows 10 or Server 2012 (& R2). I’ve found that installation files are usually not on an image I’m working with and there-in lies the problem. The next piece that throws me off is the lack of CD or DVD resources. Simply downloading .net only gives you the installer file, which does not work.

so first, get the files needed (\sources\sxs\microsoft-windows-netfx3-ondemand-package.cab)

Copy those files to a USB thumb drive (or capture an image)

Then from an elevated command prompt, run:

Dism /online /enable-feature /featurename:NetFX3 /All /Source:D:\sources\sxs /LimitAccess

replacing D: with the appropriate drive letter, of course