[FrostKiwi]

Many pieces of circuitry on different scales are already 3D. On the big side, PCBs are printed in multiple layers, going smaller we have 3D NAND flash, micro processors are very much printed in "layers" with components being connected "one story up" and those layers are again connected "two stories above". Finally you might argue on the atomic scale, individual transistors are very much 3D as well, with how source, gate and drain are constructed. But I get your point, you can't just stack those layers ad infinitum.