We study the asymptotic distribution of the resonances near the Landau levels Λq=(2q+1)b, q∈N, of the Dirichlet (resp. Neumann, resp. Robin) realization in the exterior of a compact domain of R3 of the 3D Schrödinger operator with constant magnetic field of scalar intensity b>0. We investigate the corresponding resonance counting function and obtain the main asymptotic term. In particular, we prove the accumulation of resonances at the Landau levels and the existence of resonance free sectors. In some cases, it provides the discreteness of the set of embedded eigenvalues near the Landau levels.Opérateurs non-autoadjoints, analyse semiclassique et problèmes d'évolutio