With the help of a DFT+U approach, we have analyzed the characteristics of a series of pure and mixed 3d transition metal oxide honeycomb monolayers of M2O3 and MM′O3 stoichiometries (M, M′ = Ti, V, Cr, and Fe) deposited on a metal substrate (Me = Ag, Au, Pt). We show that the substrate-induced structural polarization, interfacial electron transfer, and oxide-metal interaction strength display general trends which are governed by the offsets between the oxide band structure and the metal Fermi level. They are the strongest for the least electronegative cations (Ti and V) deposited on supports with the largest work functions (Au, Pt), where the depletion of purely 3d Ti and V states provokes an increase of the cation oxidation state. Mixing generally induces electron transfers from the least (Ti and V) to the most (Cr and Fe) electronegative cations. However, the systematic delocalization of the Ti2O3 d valence electrons to the metal substrates limits any significant mixing-induced electronic rearrangements to the V-based compounds only. We show that these electronic effects directly impact the energetics of cationic mixing and are responsible for a dramatic destabilization of the mixed TiFeO3 monolayers compared to the bulk ilmenite phase, while they stabilize ordered mixed VFeO3 films which have no bulk equivalent. Our findings give general guidelines on how oxide electronic, magnetic, and reactivity characteristics can be efficiently engineered by tuning the oxide stoichiometry and the metal substrate, of direct interest for modern technologies