This old version of .NET is often required to be installed prior to installing some piece of software or other. I’ve found that even if I download the installation file from microsoft.com, it still requires an internet connection during the installation, which isn’t always available (e.g. if I’m installing in a non-internet connected lab environment).
The framework is on the Windows 10 media, so you can install it from there by running the following command from a command line (as administrator):
dism /online /enable-feature /featurename:NetFx3 /All /Source:F:\sources\sxs /LimitAccess
Where source is the location of the sxs sub-directory on your attached Windows 10 media (in the case above, mounted to the F: drive).
This will install the framework.