We consider the 3D Schrodinger 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 $Omega_{rm in} subset {mathbb R}^3$. We introduce the Krein spectral shift functions $xi(E;H_pm,H_0)$, $E geq 0$, for the operator pairs $(H_pm,H_0)$, and study their singularities at the Landau levels $Lambda_q : = b(2q+1)$, $q in {mathbb Z}_+$, which play the role of thresholds in the spectrum of $H_0$. We show that $xi(E;H_+,H_0)$ remains bounded as $E uparrow Lambda_q$, $q in {mathbb Z}_+$ being fixed, and obtain three asymptotic terms of $xi(E;H_-,H_0)$ as $E uparrow Lambda_q$, and of $xi(E;H_pm,H_0)$ as $E downarrow Lambda_q$. The first two terms are independent of the perturbation while the third one involves the {em logarithmic capacity} of the projection of $Omega_{rm in}$ onto the plane perpendicular to $B$.
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