Evolution of the critical argument of the outer planet pair (b and e) initially locked in a 3:2 resonance. In the upper panel, the additional planet has a mass m = 200 M_⊕, a = 8 au, e = 0.95, and i = 10°. In the bottom panel, a more massive planet on a less eccentric orbit was adopted (m = 2 M_J, a = 4 au, e = 0.85, and inclination i = 2°). In both cases, after a period of chaotic evolution, the onset of repeated close encounters breaks the resonance lock and leads to fast instability.