This study investigates the three-dimensional channel flow with sudden expansion with an expansion ratio (ER = expanded channel height/inlet height of the jet) equal to three using numerical simulations in the low Reynolds number (Re) regime. The aim is to explore the flow structure variation with Re. The first critical Re (Re_c1) is observed when the equal-sized recirculation bubble on both sides of the channel becomes unequal in size. In other words, the symmetry-breaking bifurcation is obtained when the flow symmetry changes into an asymmetrical flow. Further, an increase in the Re displays a third recirculation eddy, named as second critical Re (Re_c2), and represents a turning point bifurcation. The third re-circulation bubble appears on the same side of the smaller eddy, where the jet adjusts its direction. As Re increases beyond a third critical Re (Re_c3), the flow demonstrates a three-dimensional instability before it shows unsteady behaviour. Breakdown of the spanwise vortical structures starts after Re_c3, where the flow starts to show unsteadiness, termed as the fourth critical Re (Re_c4). Interestingly, the instability in the flow first occurred at a downstream point and progressively propagated upstream towards the inlet as the Re was further increased. The turbulence levels increase with Re beyond Re_c4. The breaking down of large vortical structures into smaller ones approaches the inlet as Re increases. Finally, the flow becomes fully turbulent with small vortical structures at Re = 800, and the big re-circulation eddy presented at the expansion region at low Re disappears completely.