Electron propagator calculations accurately account for the photoelectron spectra of diborane, digallane and gallaborane. Whereas electron correlation corrections to canonical, Hartree-Fock orbital energies are necessary for accurate results, large pole strengths confirm the qualitative validity of the Koopmans description of the first five cationic states. Only for the sixth final state is there evidence of significant multiconfigurational character. Dyson orbitals corresponding to each ionization energy are dominated by a single, Hartree-Fock orbital. The order of final states and the phase relationships between atoms in the Dyson orbitals are conserved for all three molecules. As the number of Ga atoms rises, ionization energies, splittings between cationic states and direct interactions between nonhydrogen atoms decrease.