We played 3 at the back as recently as the last game vs Wolves. When Walker-Peters went off, Hasenhuttl switched to 3 central defenders (Salisu, Vestergaard & Bednarek) and 2 wing backs (Bertrand & Djenepo). The commentators didn't realise the formation change but it was pretty clear that Bednarek was not playing right back, he was on the right of a three.
Very late in the game Vestergaard moved up front so the formation was all over the place by that point.