Abstract The diamagnetic response of superconducting granular systems is studied by means of a network of Josephson junctions (JJs) and inductances. We study the diamagnetic properties of the simplest possible granular superconductor: a three-grain system which reduces to a superconducting loop closed by three JJs. We numerically determine: the irreversibility line of the system in the H vs T plane and the temperature dependent lower threshold field Hgc1(T). In addition, we remark that the metastable shielding states realized above a thermal activation line in the H vs T diagram decay in experimentally detectable times toward thermodynamic equilibrium. Finally, we mention how this simple model can be extended to more complex systems.