There is already a lot of information on this topic. Building a reference image including .NET Framework features installed. The thing is, there is no real one way to go. So combining some solutions together I came up with the following.
For installing the .NET Framework features, Windows needs additional sources located on the Windows installation media (ISO). Look for a folder called sxs. (drive:\sources\sxs)
Copy this sxs folder and create a ConfigMgr Package, with no program and distribute the content.
In the Task Sequence go to Add - General - Run Command Line
The most important is to add the Package and specify the source option in the command.
DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /LimitAccess /Source:"."
Bacause the working folder will default to the package location a dot (".") is sufficient to specify.
This will save me a lot of time in future deployments.