We report on the fabrication of diamond-like photonic structures in PMMA resist and their use as porous templates for transferring three-dimensional patterns to metals or dielectrics. Following the original ''three drilling holes'' approach first proposed by Yablonovitch, we used three consecutive exposures of PMMA resist to an x-ray beam through a triangular lattice of holes. A submicronic patterning was thus obtained in thick PMMA layers (>6 mum). Optical characterizations of 1.3 mum period templates showed a well-defined photonic gap in the midinfrared. The pattern transfers from the PMMA templates to a metal (copper) and a high refractive index dielectric (titania) were achieved by the electrodeposition and sol-gel filling techniques, respectively. Three-dimensional metallic structures of 1.3 mum lattice constant were obtained with extreme regularity over a thickness of similar to6 mum, thereby providing a way to build submicrometer photonic band gap materials for optical wavelengths.