We consider the 3D Schrödinger operator H 0 with constant magnetic field B of scalar intensity b > 0, and its perturbations H + (resp., H − ) obtained by imposing Dirichlet (resp., Neumann) conditions on the boundary of the bounded domain Ω in ⊂ R 3 . We introduce the Krein spectral shift functions ξ(E; H ± , H 0 ), E ≥ 0, for the operator pairs (H ± , H 0 ), and study their singularities at the Landau levels Λ q := b(2q + 1), q ∈ Z + , which play the role of thresholds in the spectrum of H 0 . We show that ξ(E; H + , H 0 ) remains bounded as E ↑ Λ q , q ∈ Z + , being fixed, and obtain three asymptotic terms of ξ(E; H − , H 0 ) as E ↑ Λ q , and of ξ(E; H ± , H 0 ) as E ↓ Λ q . The first two terms are independent of the perturbation while the third one involves the logarithmic capacity of the projection of Ω in onto the plane perpendicular to B.