You don't say the hive has no brood but I'm assuming that's what you meant. If so, and it's been 3 weeks, probably best to combine it with another. That way you avoid the risks of attempting to requeen a hive that has been in a questionable state for 3 weeks, plus the bees can get right to work supporting the new queenright hive. I would recommend placing a queen excluder between the two hives for the first couple of weeks, just on the off chance that the queenless hive has a virgin or something that may kill your good queen. Unlikely, but does happen from time to time.
If you wanted the extra hive, you could get a new queen some time down the track and re-split this hive.
If you wanted the extra hive, you could get a new queen some time down the track and re-split this hive.