That's a proper unsettling one mate. The three nights in a row thing is what gets me, because random one-off sightings are easy to dismiss but when something keeps coming back on a pattern like that it feels way more deliberate somehow.
Had something similar in my old house in Matlock a couple years back, kept seeing something at the end of the landing around 2-3am. Never fully formed, just this dark mass that was definitely darker than the surrounding darkness if that makes sense. Took photos a few times but nothing showed up.
Did yours have any particular time it appeared? And was the shape consistent or did it change? Also worth asking - had anything significant happened in the days before the first sighting, like moving furniture around or any kind of emotional stress in the house? In my experience poltergeist and shadow activity tends to spike around disruption of some kind. Would love to hear more detail on this one.