Lisa: You mentioned Apistan - Good!  You mentioned Terramycin - Why?  But you
did mention Menthol, the ONLY approved killer of the Tracheal mite.
What month did your bees die?  If you found a hive with zero or just a few
dead bees in it, yet there was plenty of honey - this is the classic evidence
of tracheal mite, KILLED ONLY BY MENTHOL.
Now you have 3 hives.  Regarding the "sick" one, have you examined INSIDE, or
are you just looking at the outside.  Maybe all the bees are dead, and what
you see going in and out are ROBBER bees.
I am trying to be nice - Nobody except God can explain the condition of a
colony FROM THE OUTSIDE - You have to examine the BROOD CHAMBER, not the
supers.
If you can tell me more, I will help you.   George Imirie