  The cross section for the photoproduction of events containing three jets
with a three-jet invariant mass of M_3J > 50 GeV has been measured with the
ZEUS detector at HERA. The three-jet angular distributions are inconsistent
with a uniform population of the available phase space but are well described
by parton shower models and O(alpha alpha_s^2) pQCD calculations. Comparisons
with the parton shower model indicate a strong contribution from initial state
radiation as well as a sensitivity to the effects of colour coherence.
