Dungeon petz 2 or 3 player game: how many cages should be replenished?
The first two sentences of that section still apply, and state


  At least one cage will be left over from the previous round. Take the
  lowest of the leftover cages, put it in the uppermost pit and discard
  the rest.


It isn't possible for all 3 cages to be purchased in a turn, so at least one will always be there.  Keep the lowest, move it to the upper slot, and get rid of the rest.  That way you are guaranteed to always have a single slot filled.  Then you either deal 2 more, or only 1 more if a neutral imp is blocking a space.