Artificial neural network circuits, including 3D ones, are nothing new of course.
But efficiently and relatively cheaply fabricating them into a 3D circuits has always been a big challenge until apparently now:

https://techxplore.com/news/2020-05-d-memristor-based-circuit-brain-inspired.html

This article doesn't call it "3D neural network" but rather for some reason actually calls it "3-D memristor-based circuit for brain-inspired computing" which, although has a very subtly different meaning from "3D neural network", in my opinion they were being far too pedantic here and they really should have just stuck to calling it a "3D neural network".