The fast thermal and electromagnetic transients that occur in a superconducting magnet in case of a quench have the potential of generating large mechanical stresses both in the superconducting coils and in the magnet structure. While the investigation of such quench loads should generally be conducted to ensure a safe operation of the system, its importance is greatly enlarged in the case of high-field magnets based on strain sensitive superconductors. The scope of this work is hence to bring, for the first time, a detailed understanding of the three-dimensional mechanical behavior of a Nb3Sn accelerator magnet during a quench discharge. The study relies on the use of Finite Element models, where various multi-domain simulations are employed together to solve the coupled physics of the problem. As a complement to the simulation analyses, our research includes as well a full experimental campaign for the electro-mechanical characterization of Nb3Sn superconductors.