The CAB file is either for the wrong architecture (e.g., amd64 on x86) or language mismatch.
Type microsoft-windows-netfx3-ondemand-package.cab into the search box. You will see a list. Look for the entry that matches your Windows version and build number. For example: Download Microsoft-windows-netfx3-ondemand-package.cab
This fails for several reasons:
: If the PC is online, simply run DISM /Online /Enable-Feature /FeatureName:NetFx3 /All . Windows will download the necessary files from Microsoft's servers. The CAB file is either for the wrong architecture (e