The cross section for the photoproduction of events containing three jets w
ith 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 descri
bed by parton shower models and O(alpha alpha(s)(2)) pQCD calculations. Com
parisons with the parton shower model indicate a strong contribution from i
nitial state radiation as well as a sensitivity to the effects of colour co
herence. (C) 1998 Elsevier Science B.V. All rights reserved.