  Principal outer automorphisms were introduced in Algom-Kfir-Kapovich-Pfaff to
emulate principal pseudo-Anosov surface homeomorphisms, i.e. those whose
attracting and repelling invariant foliations have only 3-pronged
singularities. It is proved in Algom-Kfir-Kapovich-Pfaff that principal fully
irreducible outer automorphisms semi-mimic principal pseudo-Anosov surface
homeomorphisms in important ways. We prove here which rank-3 graphs carry train
track maps for principal fully irreducible outer automorphisms. As a corollary,
one obtains which rank-3 Culler-Vogtmann Outer space simplices are passed
through by principal axes.
