All three bosses can be found in the Drakkar Lake area. Koren Wildrunner patrols near the frozen lake, and is near Myish, Lady of the Lake who will be on the lake itself (surrounded by many Chilling Wisps). Like many EotN mobs, the Wisps follow a long patrol route, so try to avoid fighting them along the path they follow. More mobs can quickly pile up. After those 3 bosses are defeated, return for your reward and the follow-up quest.