This trio is NPC enemies located in Grand Archives on a staircase that leads to the Twin Princes bossfight.
You are the third Black Hand.
One of them threw themselves down the elevator shaft when I was going down
Why are they called "Black Hand Trio"? Kamui is the only Black Hand here... In the first place it is confirmed that there're only three black hands within the entire duration of the Lothric kingdom. Adding Kriemhild and Albert would make AT LEAST 4 Black Hands ( i say 4 instead of 5 because the 3rd Black Hand is never seen and could be either of these two if they took off their armor, though i doubt so since they all seem to dual wield). I vote they're re-named to "DS3 Gank Squad" (DS3 has to be included in the name because DS2 had it's own gank squad), since it is literally just inaccurate to call the other two Black Hands.