A prominent class of refractory ceramics, transition metal diborides, has recently attracted scientific attention – both in their bulk and thin film form – due to their high-temperature stability and extreme mechanical properties. The thin film community has primarily focused on metastable solid solutions, where alloying with silicon leads to massively enhanced oxidative properties combined with only minor drawbacks in hardness. However, theoretical investigations of TM-Si-B2 based systems have been limited until now. Therefore, this study employs ab initio methods to shed light on the physical fundamentals of metastable solid solutions of TiB2, ZrB2, and HfB2 with silicon. Utilizing Density Functional Theory (DFT), the structural and mechanical properties were investigated and compared well with experimental thin films. Methods such as Crystal Orbital Hamilton Populations (COHPs) allowed to couple chemical stability and elemental bonding properties, whereas Radial Distribution Functions (RDFs) and simulated XRD patterns unraveled a direct connection between experimentally explored Si solubility limits and AlB2 type structure loss. Moreover, a novel ab initio-based statistical technique was explored to investigate maximal solubilities in the metastable solid solution state. Furthermore, a structural instability of ternary diborides concerning transition metal vacancies was found, in contrast to low variations of the chemical stability at high silicon contents.