We have designed and developed a simplified three-dimensional 3D photonic crystal PhC fabrication technique that can be used to fabricate a nanoscale 3D structure from the two-dimensional 2D surface of a silicon Si, or silicon on insulator SOI wafer with a single modified Bosch plasma etching process. Using this technique, we demonstrated such a PhC structure that includes hollow-core waveguides with high-contrast gratings as cladding. The etching process produces deep trenches with controlled width variation along the vertical direction. This method uses only a single mask such as e-beam lithography mask without alignment, and there is no need for deposition, regrowth, etc. This technique may greatly reduce the fabrication cost and increase the yield of 3D PhC devices.