Differential three-jet cross sections have been measured in e+e--annihilation at an average CM energy of 33.8 GeV and were compared to first- and second-order predictions of QCD and of a QED-like abelian vector theory. QCD provides a good description of the observed distributions. The inclusion of second-order effects reduced the observed quark-gluon coupling strength by about 20% to αS = 0.16 ± 0.015 (stat.) ± 0.03 (syst.). The abelian vector theory is found to be incompatible with the data.
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