You need to provide enough current to power all the devices. The USB port provides 5 volts. So assuming a realistic worst case scenario you want 2 amps or 10 watts per device - so for 4 devices you need a power supply for the hub which can output 8 amps @ 5 volts or 40 watts.
Your actual requirement will probably be something less depending on your hard drives. 2.5 inch 5400 rpm drives will typically use around 1amp with 7200 rpm drives using more - and 3.5 inch drives will use very little power from the hub as they draw too much power to be powered off USB.
You mentioned a RPI3B - be aware that your speeds will suck. While I'm sure it can be done in theory, I can't see this setup running RAID5 or RAID6 with meaningful speed, but then again, with its 100Base Ethernet port its not going to give you more then 12 megabytes per sec best case scenario anyway (less then 1/10th the speed of a typical hard drive).
(What I'm trying to say, politely, is that this may be doable in theory but will be frustratingly slow in practice. There are most likely better solutions which suck less for not a lot more money. I've not researched much but even a Pine64 will give you a gigabit network card and USB3 interface which at least gives you a shot at usable speeds )