1. From the entrance to the dungeon, go right along the road and slightly upwards, and you'll find a garrison.
2-3. From the central Necropolis castle, go right along the road for quite a while, and then down along with it, and there should be two garrisons.
4. From the entrance to the dungeon, go left-down along the road, and there will be a fork somewhere down from it, and the third garrison will be there...
Sorry for the rambling answer, I just haven't played it in a long time... maybe someone remembers better, let them chime in...