I'm not sure how you would go about kicking off a download, but you can get the files from the Windows 10 install CD/ISO. Copy the folder called 'D:\sources\sxs' and store these file somewhere.
Once you have the files you can install them with the following command, run it with administrative rights. Make sure you change the /Source: parameter to the location you copied 'sxs' folder to.
DISM /online /enable-feature /featurename:NetFx3 /All /Source:D:\sources\sxs /LimitAccess
If you wish to unstill the feature by command line you can use the following comamnd.
DISM /online /disable-feature /FeatureName:NetFx3
