Hydrogen blending and biogas upgrading are usually treated as separate routes for decarbonizing gas networks. This work investigates their direct combination through a nonlinear optimization framework for the compositional design of natural gas (NG), hydrogen, and pre-treated, non-upgraded biogas blends under coupled gas quality and combustion screening constraints, followed by a transient hydraulic assessment. Two biogas qualities (75% and 55% CH₄) and two objectives (minimum residual NG and maximum hydrogen share) are examined while the admissible bounds are progressively relaxed. The CO₂ retained in biogas moderates the reactivity increase associated with hydrogen, while hydrogen offsets the density penalty of CO₂. Under the current bounds, ternary blending raises the admissible renewable share from about 2% (binary NG–H₂) to 10–17% by volume. A 10% relaxation defines the most relevant operating region: renewable gas reaches almost 40% by volume (32% on an energy basis) for the higher-quality biogas and about 23% for the methane-lean one, with laminar burning velocities about 5% lower than NG and moderate hydraulic and cost penalties. Complete NG displacement requires 34% and 56% relaxation and causes severe quality and transport penalties. This framework can support gas quality planning by network operators, the valorization of small biogas plants without upgrading, and regulatory scenario analysis.