  We theoretically study the superconducting diode effect in a three-terminal
Josephson junction. The diode effect in superconducting systems is typically
related to the presence of a difference in the critical currents for currents
flowing in the opposite direction. We show that in multi-terminal systems this
effect occurs naturally without the need of the presence of any spin
interactions and is a result of the presence of a relative shift between the
Andreev bound states carrying the supercurrent. On an example of a
three-terminal junction, we demonstrate that the non-reciprocal current in one
of the superconducting contacts can be induced by proper phase biasing of the
other contacts, provided that there are at least two Andreev bound states in
the system and the symmetry of the system is broken. This result is confirmed
in numerical models describing the junctions in both the short- and
long-regime. By optimizing the geometry of the junction, we show that the
efficiency of the realized superconducting diode exceeds $35 \%$. We relate our
predictions to recent experiments on multi-terminal junctions, in which
non-reciprocal supercurrents were observed.
