We show that most of the genus-zero subgroups of the braid group B 3 (which are roughly the braid monodromy groups of the trigonal curves on the Hirzebruch surfaces) are irrelevant as far as the Alexander module is concerned. There is a very restricted set of subgroups, which we call "primitive", such that these subgroups and their intersections determine all the Alexander modules. Then, we classify the primitive subgroups of genus zero which belong to a particular kind and compute their Alexander modules. This result implies, in particular, the known classification of the dihedral covers of the trigonal curves.