Transition metal diborides, a class of refractory ceramics, are well known for their high-temperature stability and extreme mechanical properties, prompting scientific interest in both their bulk and thin film forms. The research community has recently focused its attention on the formation of metastable solid solutions with silicon, with a view to enhance the oxidative properties and fracture characteristics. However, theoretical investigations of such ternary compounds remain sparse. Therefore, the current study employs Density Functional Theory (DFT) to explore structural, energetical, and mechanical properties of the Ti-Si-B2, Zr-Si-B2, and Hf-Si-B2, respectively. Furthermore, the structural and chemical stability, with respect to defected structures, was investigated. By analyzing Radial Distribution Functions (RDFs) and simulated XRD patterns, a comparison with experimental thin film XRDs and compositions validated proposed solubility limits. The structural chemistry, unraveled through investigating the compounds Crystal Orbital Hamilton Populations (COHPs), attributed the loss of AlB2-type symmetry to Si clustering and anti-bonding interactions of boron with silicon. Simulated elastic properties reproduced experimental values up to 15 at. % Si, whereas a structural instability of ternary AlB2-type compounds, with respect to metal vacancies, was found. However, the introduction of both metal or boron vacancies showed a diminishing impact on the chemical stability, at elevated silicon contents.