A certified partial census of bistability in three-species open bimolecular mass-action networks with at most three internal reactions

We enumerate all mass-action networks on three species built from complexes of molecularity at most two, with at most three internal reactions and an inflow and a linear removal for every species: 62,269 labelled networks, 10,523 up to species relabelling. 754 networks carry an exact rational bistability witness (two distinct positive rational equilibria, both exactly Routh–Hurwitz stable), independently re-verified by an auditing AI agent; 5,071 carry a certified exclusion (at most one positive equilibrium, by a determinant-sign criterion of Craciun–Feinberg type proved here for quadratic fields by a midpoint identity); 807 have two exact equilibria but no stable pair found (not independently verified); 3,891 (37.0%) are UNDECIDED. The census is partial; an undecided network is search evidence only. Among connected-support networks 727 are bistable, 175 minimal, of which 79 contain a never-reactant species (effectively two-species cascades) and 96 are irreducible by our screen; exactly nine networks with two internal reactions are bistable and are listed with exact witnesses. The census reproduces Peng's two-species classification (arXiv:2610.32739, 27 of 681), which is Peng's result. No claim of new mechanisms and no biological claim; novelty is relative to the cited literature, to our knowledge only. Producer and auditor are AI agents; no human audit; the preregistration hash is self-attested; no Lean.