This study investigates the hydrogen-assisted crack propagation mechanisms in FeNiCr medium-entropy alloys (MEAs) through molecular dynamics (MD) simulations and modeling, focusing on the roles of hydrogen concentration, chemical short-range ordering (CSRO), crystallographic orientation, and loading rate. By extending the Rice-Thomson framework, we demonstrate that hydrogen increases the energy barrier for dislocation emission, suppressing crack-tip plasticity. Compared to random solid solutions, CSRO-structured FeNiCr alloys exhibit a higher critical stress intensity factor for dislocation nucleation and promote hydrogen segregation near the crack tip to form hydrogen atmosphere. At lower hydrogen concentrations, hydrogen facilitates dislocation glide, consistent with the hydrogen-enhanced localized plasticity (HELP) mechanism. At elevated concentrations, however, hydrogen atmosphere strongly pins dislocations, leading to markedly irregular dislocation lines.