Hi Constantin,

Well, it sounds like robbing.  Try reducing your entrances.  I'm not
familiar with conditions in Romania but I assume most of the main honey
flows are over?  You should try to find out why the hives are weak and
deal with it from that angle .  Maybe disease, mites or a poor queen.
The one hive sounds bad.  To much depends on the conditions in your area
so I really don't feel I can tell you for sure what to do regarding the
one weak hive.  If there is still ample time left before it gets to cold
you could try to buy a queen.  I think it might be to late for you to
raise one yourself, since it takes about a month to get a laying queen
and you need the drones to,  but you will know better yourself.  If you
have doubts and the problem persists then I think if it was me I would
unit the hive with another and hope for the best.  Just make sure you
kill the queen you don't want first. Good luck and keep us posted!

Kent