Multiple usb devices attached to airport extreme

Sure, but keep in mind that the port on the AirPort is an older USB2.0 design, and the processing power of the AirPort is limited to about half of normal USB2.0 speeds, so things are going to be on the slow side.

Make sure that the hub has its own power supply since the port on the AirPort is underpowered.

The hard drives need to be formatted in Mac OS Extended (Journaled) to be compatible with the USB port on the AirPort as well.

In theory, yes. USB 3.0 is supposed to be backwards compatible with USB 2.0.

If you can find a USB 2.0 hub, I would go for that model.