  We present a measurement of the ratio of multijet cross sections in pp-bar
collisions at sqrt(s) = 1.96 TeV at the Fermilab Tevatron Collider. The
measurement is based on a data set corresponding to an integrated luminosity of
0.7 fb-1 collected with the D0 detector. The ratio of the inclusive three-jet
to two-jet cross sections, R3/2, has been measured as a function of the jet
transverse momenta. The data are compared to QCD predictions in different
approximations. Popular tunes of the PYTHIA event generator do not agree with
the data, while SHERPA provides a reasonable description of the data. A
perturbative QCD prediction in next-to-leading order in the strong coupling
constant, corrected for non-perturbative effects, gives a good description of
the data.
