Droplet-based microfluidic devices provide a powerful platform for material, chemical and biological applications based on droplet templates. The technique traditionally utilized to fabricate microfluidic emulsion generators, i.e. soft-lithography, is complex and expensive for producing three-dimensional (3D) structures. The emergent 3D printing technology provides an attractive alternative due to its simplicity and low-cost. Recently a handful of studies have already demonstrated droplet production through 3D-printed microfluidic devices. However, these devices invariably use purely two-dimensional (2D) flow structures. Herein we apply 3D printing technology to fabricate simple and low-cost 3D miniaturized fluidic devices for droplet generation (single emulsion) and droplet-in-droplet (double emulsion) without need for surface treatment of the channel walls. This is accomplished by varying the channel diameters at the junction, so the inner liquid does not touch the outer walls. This 3D-printed emulsion generator has been successfully tested over a range of conditions. We also formulate and demonstrate, for the first time, uniform scaling laws for the emulsion drop sizes generated in different regimes, by incorporating the dynamic contact angle effects during the drop formation. Magnetically responsive microspheres are also produced with our emulsion templates, demonstrating the potential applications of this 3D emulsion generator in chemical and material engineering.