Thanks for your help! I tried using it and now I’m stuck at the easy part already. The List.IndexOf node retrieves -1 for all Indexes and the List.GetItemAtIndex can’t get those items. I thought about solving it like this, but sadly it didn’t work yet:
@Daan described this here IndexOf Node Issue (Returning -1) - #2 by AmolShah earlier in 2020 and it made sense for me, but not for the program
When used with only three points the delaunay node retrieves null 
You can download my Revit file here: Onedrive Link for Lightning.rvt