  We consider the 3D Schr\"odinger 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 $B$.
