Your question is not very clear (with respect of your OS or what you are trying to do). You can RAID HDD's over USB under Linux using software raid (MDADM)
I would imagine you would not generally want to RAID drives using USB2 or slower because performance will suck rocks. Theoretically USB3 should work fine, but as you are abstracting away from the SATA (or whatever) Interface they are unlikely to perform quite as well.
Having each device on its own USB3 port would give you better performance then having them go through a hub sharing a single USB port as there is more available to each device that way.
With USB, you would need a hub to manage the traffic. USB 3.0 speed is roughly comparable to SATA, so sharing a single connection, even with USB 3.0, would not have the bandwidth to keep up with what the drives are capable of. – fixer1234 – 2015-01-03T08:31:41.137