A method is for forming three-dimensional micro- and nanostructures, based on the structuring of a body of material by a mould having an impression area which reproduces the three-dimensional structure in negative form. This method includes providing a mould having a substrate of a material which can undergo isotropic chemical etching, in which the impression area is to be formed. An etching pattern is defined on (in) the substrate, having etching areas having zero-, uni- or bidimensional extension, which can be reached by an etching agent. A process of isotropic chemical etching of the substrate from the etching areas is carried out for a corresponding predetermined time, so as to produce cavities which in combination make up the impression area. The method is advantageously used in the fabrication of sets of microlenses with a convex three-dimensional structure, of the refractive or hybrid refractive/diffractive type, for forming images on different focal planes.