If you have your Windows installation media, CD or DVD, you can type this into a Command Prompt.
DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /LimitAccess /Source:D:\sources\sxs
Where you see "D:\sources\sxs", replace the drive letter with where your Windows USB or CD drive is.
If you use Windows 10 and don't have installation media, you can make a USB or CD with the
Media Creation Tool first, then try the above.