Licentia poetica. Something like a molecule drawn from memory without understanding what it really is. I believe these 3N marks don't mean anything, the only thing important is a structure/skeleton (and whoever painted them didn't know why in the original some atoms were marked as N and some were not marked at all).
Edit:
why would they be marked N (3N?) if they are only carbon?
I don't think they are ONLY carbon, I think most of them are carbon, but some can be N or O.