  We study three-terminal linear and nonlinear transport through an
Aharonov-Bohm interferometer containing a double quantum dot using the
nonequilibrium Green's function method. Under the condition that one of the
three terminals is a voltage probe, we show that the linear conductance is
symmetric with respect to the magnetic field (phase symmetry). However, in the
nonlinear transport regime, the phase symmetry is broken. Unlike two-terminal
transport, the phase symmetry is broken even in noninteracting electron
systems. Based on the lowest-order nonlinear conductance coefficient with
respect to the source-drain bias voltage, we discuss the direction in which the
phase shifts with the magnetic field. When the higher harmonic components of
the Aharonov-Bohm oscillations are negligible, the phaseshift is a
monotonically increasing function with respect to the source-drain bias
voltage. To observe the Aharonov-Bohm oscillations with higher visibility, we
need strong coupling between the quantum dots and the voltage probe. However,
this leads to dephasing since the voltage probe acts as a B\"{u}ttiker
dephasing probe. The interplay between such antithetic concepts provides a peak
in the visibility of the Aharonov-Bohm oscillations when the coupling between
the quantum dots and the voltage probe changes.
