Loc is correct. The 3 hoarders spawn uncommonly after killing mobs in that room. Highly recommend keeping the room fully clear rather than only killing the spawn marked as named on map. Often I'd get the hoarders to spawn after killing the last mob in room, but not always. Can certainly trigger after killing one mob, just less common. Took well over 12 hours, saw 7 or so sets of 3 hoarders before I got the treasurer to spawn.
As other comments have mentioned this mob pops in the room at 1750, -450, the same room as "the Broken Skull Defender". Every so often there is a little mini-event that will happen where 3 extra mobs will spawn in the room, either 3 "a Broken Skull hoarder" or 2 hoarders and the treasurer. I don't know what exactly controls how often this event goes off but in the span of about 10 hours I had it happen twice, the first time I got 3 hoarders and the second time the treasurer took one of their spots.
Torgiran Mines, treasurer spawned in the room at (+1750, -470) after killing a bloodguard. From Nadox, go through hidden wall and keep going straight through the door, turn left and go down ramp through next door, go straight to end of hall and turn left at the T, room is a dead end on the right. Don't know if it can spawn in other rooms or not.