For Win8 execute from elevated prompt where d:\ is your Windows 8 installation disk.
dism /online /enable-feature /featurename:NetFx3 /all /source:d:\sources\sxs
For Win10 execute from elevated prompt.
DISM /Online /Enable-Feature:NetFx3 /All
For Win8 execute from elevated prompt where d:\ is your Windows 8 installation disk.
dism /online /enable-feature /featurename:NetFx3 /all /source:d:\sources\sxs
DISM /Online /Enable-Feature:NetFx3 /All