Selective area growth of zinc phosphide (Zn3P2) on InP provides a pathway to high-quality semiconductor nanostructures and textured thin films made of earth-abundant elements. In the precoalescence stage, Zn3P2 emerges in the form of nanoislands undergoing a peculiar shape transformation in the course of growth. We present a model based on the minimization of the surface energy with respect to the relevant geometrical parameters which quantitively describes the shape of nanoislands depending on their volume. The results are presented in the dimensionless variables which allow us to comprehend simultaneously the islands grown in differently sized pinholes and for different growth times. The shape transformation is driven by a competition of (112) and (101) side facets and (001) top facet. The islands are flat and regular octagonal at the beginning of growth, transitioning to the full nanopyramid restricted solely by (101) facets at the end.