In-grown group III (cation) vacancies (VGa, VAl, VIn) in GaN, AlN and InN tend to be complexed with donor-type defects These donor defects may in principle be residual impurities such as O or H, n-type dopants such as Si, or intrinsic defects such as the N vacancy (VN). The cation vacancies and their complexes are generally deep acceptors, and hence they compensate for the n-type conductivity and add to the scattering centers limiting the carrier mobility in these materials. Mg doping reduces the group III vacancy concentrations, but other kinds of vacancy defects emerge. This work presents results obtained with positron annihilation spectroscopy in GaN, AlN, and InN. The vacancy-donor complexes are different in these three materials, and their importance in determining the opto-electronic properties of the material varies as well. The formation of these defects is discussed in the light of the differences in the growth methods.