Run a command prompt as an administrator and use:
dism /online /Get-Features
This will display the feature names since they don't always match up with what you're seeing in that visual feature list. It will also show which are currently enabled/disabled. Once you find the feature that you'd like to enable (NetFx3 in this case), run this:
dism /online /Enable-Feature /FeatureName:NetFx3
And as Richard stated, you can then disable a feature by simply switching "Enable" to "Disable" ex.
dism /online /Disable-Feature /FeatureName:NetFx3
Note: Sometimes a restart is required to see changes with windows features.