A novel form of Terzaghi's effective stress principle is proposed to describe electro-chemo-mechanical couplings in unsaturated swelling clays characterized by two porosity levels (nanopores saturated by an aqueous electrolyte solution and micropores partially saturated with water and air). Under mechanical equilibrium, the three-scale model is derived using the homogenization procedure. Macroscopic constitutive laws are derived within the framework of work-conjugated stress-strain variables leading to a set of three pairs of state variables. In addition to the contact stress between particles and a Bishop component-type related to capillarity effects along with the skeleton strain and water saturation, the novel form includes salinity as an additional strain variable and a three-scale swelling stress directly correlated with the electrochemical phenomena at the nanoscale. Numerical results illustrate the potential of the multiscale approach in capturing the complex features of unsaturated expansive clays.