The aim of this thesis is to present a comprehensive study of the effect of three-dimensional stresses on industrial A15 superconductors, from the microscopic and macroscopic point of views. The effects of stress and strain on the Nb3Sn crystallography have been described by means of a high-energy X-ray diffraction experiment on wires under axial strain. A direct relationship between the restoring of the cubic shape of the Nb3Sn lattice cell and the maximum value of critical current as a function of axial strain for a Nb3Sn wire is shown. The different responses of the critical current to the axial tensile and compressive strain for the most used fabrication technique are shown. The effect of transverse pressure on the critical current of A15 superconductors has been discussed. A new setup developed at the University of Geneva in collaboration with CERN has been described: this device allows the simulation of the operating conditions for wires in accelerator magnets. A part of this thesis is dedicated to the fabrication of high performance Nb3Sn wires.