Note: The spawn of the named works the same way for all three:
first "wave" of mob's in the room are apprentice skeletons. If you kill them you get a chance to spawn the next "higher" type (unfortunately dont know the english name, "Ersatz-" in German). Everytime you clear that wave you get a chance on the next (third) level of that wave. Those are "angry" apprentice skeletons. Killing that third wave gives you a chance to spawn the named in the little chamber. I heard that the chance is about 33% to get "up".
Spawn time is 10 minutes. Ff you are high enough in level, clean all 3 rooms in a row and you will have them up again.