The flow downstream of a three-dimensional double backward facing step (3D DBWFS) is investigated for Reynolds number Re h ranging from 5 × 10 3 to 8 × 10 4 (based on the first step height h). The flow is studied both qualitatively by means of laser tomoscopy and oil-flow visualizations and quantitatively by means of particle image velocimetry (PIV) measurements. In particular, the results show a mean flow asymmetry. A sensitivity study around zero degree sideslip has shown that the flow is bistable for this geometry. This bistability has been observed in two different wind tunnels for very different upstream conditions. As a main consequence, the zero degree drift angle could be a relevant validation case of unstable flow computation. More tests are carried out to understand and control this particular flow feature.