This work focuses on the fabrication of three-dimensional (3D) microstructures by electrochemical etching of n-type silicon. We exploit the possibility of modulating the pore diameter along the growing direction to produce quasi-3D structures which exhibit periodicity also in third dimension. The fabricated structures are further processed to convert them into truly 3D networks consisting of interconnected silicon veins embedded in air. In particular, we show how a variety of complex 3D networks can be obtained starting from a simple initial pore arrangement, e.g. square or hexagonal. These 3D structures can be easily fabricated with very high precision and uniformity on a large scale. They constitute ideal hosts for sensing applications as well as perfect molds for casting 3D structures of unusual materials.