This should work.
I don't have a multibay enclosure, but have had 3 hard drives connected via a powered USB hub to the AirPort Extreme with no problems in the past.
I'm not sure if you are planning to use Time Machine for backups, but if you are....you should know that Apple does not support this type of configuration.
It might work, or you might have issues. Bottom line is that it is not a reliable method for Time Machine backups.