  We present a complete calculation of the electroweak O(alpha^3 alpha_s)
corrections to three-jet production and related event-shape observables at
electron--positron colliders. The Z-boson resonance is described within the
complex-mass scheme, rendering the calculation valid both in the resonance and
off-shell regions. Higher-order initial-state radiation is included in the
leading-logarithmic approximation. We properly account for the corrections to
the total hadronic cross section and for the experimental photon isolation
criteria. To this end we implement contributions of the quark-to-photon
fragmentation function both in the slicing and subtraction formalism. The
effects of the electroweak corrections on various event-shape distributions and
on the three-jet rate are studied. They are typically at the few-per-cent
level, and remnants of the radiative return are found even after inclusion of
appropriate cuts.
