As this is a very common ‘issue’ I am just going to post the quick-fix:

If you have internet:

DISM /online /enable-feature /featurename:netfx3 /all

Install from install media:

# Where 'd' is the drive the install media is mounted to. You might have
# to change it if your media is on a different drive letter
DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /LimitAccess /Source:d:\sources\sxs