  Two-terminal conductance spectroscopy of superconducting devices is a common
tool for probing Andreev and Majorana bound states. Here, we study
theoretically a three-terminal setup, with two normal leads coupled to a
grounded superconducting terminal. Using a single-electron scattering matrix,
we derive the subgap conductance matrix for the normal leads and discuss its
symmetries. In particular, we show that the local and the nonlocal elements of
the conductance matrix have pairwise identical antisymmetric components.
Moreover, we find that the nonlocal elements are directly related to the local
BCS charges of the bound states close to the normal probes and we show how the
BCS charge of overlapping Majorana bound states can be extracted from
experiments.
