Electronic properties of small pyramidal clusters of Ga, In and As atoms are investigated using a multi-configuration self-consistent field approximation (MCSCF). The structure and composition of some of these (pre-designed) clusters have been derived from those of the symmetry elements of the corresponding bulk fcc lattices, to reflect the cluster formation in confinement or on surfaces. The counterpart (vacuum) clusters obtained without any spatial constraints applied to the atomic positions have also being synthesized computationally. It has been shown that manipulations with the cluster structure, composition and spatial constraints permit to engineer virtual (i.e., fundamental theory-based, computational) clusters with pre-designed electronic properties that can serve as prototypes for nanoscale heterostructure (NHS) units. Computational data so obtained are in agreement with existing experimental evidence concerning such pyramidal clusters.