Ongoing and future efforts to curb carbon dioxide emissions in fulfilling energy needs involve the use of carbon-free energy carriers such as hydrogen and ammonia. Motivated by the renewed interest in ammonia, several research groups have recently investigated the premixed combustion characteristics in laminar and turbulent flames comprised of NH3/H2/N2-air fuel-air blends, generated from partial ammonia decomposition. An interesting and rather unique aspect of these NH3/H2/N2-air blends, beside their obvious relevance to carbon-free industrial applications, is the fact that their nominal unstrained laminar premixed flame properties (for combustion in air) can be tailored to match those of methane-air premixed flames over a wide range of equivalence ratios, preheat temperatures and pressures. This affords an opportunity to construct an interesting comparison between nominally identical flames that differ solely by the molecular-diffusion characteristics of the fuel, something that is not easily achievable in hydrogen-enriched methane-air or syngas-air premixed flames. Direct Numerical Simulations (DNS) are used to gain insight on the fundamental characteristics of NH3/H2/N2-air flames and reveal the key role of molecular diffusion of light fast-diffusing species as molecular and atomic hydrogen in significant enhancement of the burning rate and resilience to blow out. Moreover, DNS results suggest that these mainly thermo-diffusive effects related to the presence of hydrogen in the burnable mixture are further enhanced at pressurized conditions.