Three-jet production is studied for the first time in deep-inelastic positr
on-proton scattering. The measurement carried out with the H1 detector at H
ERA covers a large range of four-momentum transfer squared 5 < Q(2) < 5000
GeV2 and invariant three-jet masses 25 < M-3jet less than or similar to 140
GeV. Jets are defined by the inclusive k(perpendicular to) algorithm in th
e Breit frame. The size of the three-jet cross section and the ratio of the
three-jet to the dijet cross section R-3/2 are described over the whole ph
ase space by the predictions of perturbative QCD in next-to-leading order.
The shapes of angular jet distributions deviate significantly from a unifor
m population of the available phase space but are well described by the QCD
calculation. (C) 2001 Elsevier Science B.V. All rights reserved.