I'll do some looking to see if any give more than 3 amps, but my hub I'd plug into it does have it's own power. Based on what you said, that may be the best solution since I already have it and it has a lot of ports (8 plus two USB ports just for charging things).
I do have a lot of stuff to plug in. For some years I've wondered about these online backup systems but, so far, haven't signed up for one. Until recently, my internet speeds, especially uploading, weren't all that fast. But now I have 100 mbs uploads. I need something that will back up both my main drive and connected drives. But I just don't know which to get. I heard some good things about all and bad too! For example, I think it was iDrive where some users said that when they wanted to cancel, it was almost impossible to do it - there was no place they could find to do it!
Anyway, while waiting on that decision, I have simply done an overabundance of external drives! Especially if they were on sale. I have a Time Capsule, so that's where my Time Machine backup is kept.
I have a 2 TB WD Passport drive divided into 2 partitions. The first is maintained by SuperDuper to be an exact copy of my internal drive (nice to have in case the internal system fails). The second contains all my iTunes stuff (that part is getting pretty full, but after I have everything set up on my Mini, I'll redo that whole thing, cutting the size of the first partition since I only am backing up 512 GB rather than 1 TB).
Then I have a couple Seagate drives, also divided into two partitions. One partition on each also includes a copy of iTunes stuff, but also some stuff that is related to media that I don't keep in iTunes or in Photos, but I don't want to discard.
Anyway, it may be overkill, but it should reduce risk! Barring something that destroys my condo!
So, in my Hub, I believe I have a dongle for my Logitech mouse, a dongle for my TB, up to 3 hard drives. Also I keep 1 (preferably 2) ports for thumb drives.
That totals, if I haven't missed anything, 7 items. There is also my printer, but normally that is operated wirelessly.
The two Seagate drives have extra USB ports on them. I haven't done much experimenting to see how well they work or what they best work with (I simply don't know, for example, if I plugged my mouse or TB dongle into them, would there be a noticeable delay in response. Also, while I did once have one seagate plugged into the other and it at least seemed fine, I have not tried adding the Passport to the chain as well (the Passport does not have an extra port so it has to be the last thing).
I definitely would think, even if one or two HD could work on that unpowered hub you linked to, three would likely be much.
I know in the past there were items which wouldn't work unless plugged directly into a computer rather than a hub. I don't know if such problems still exist, but because of that experience, I like to not use up all my internal ports. Examples of things I remember not working were a printer and mouse dongle. Interestingly, that same printer did work without direct connection when I bought by first MacBook in 2008 (plugged it into the Time Capsule I had at the time). I think the mouse I had then may not have. But it does with my iMac in Mini.
Trying to evaluate the big picture, I'm thinking the adapter, used in conjunction with my powered hub, may be the best step. Especially since, over time, I'm likely to do upgrades that are USB-C/Thunderbolt. Like future external SSD drive. I don't know if a USB-C connected regular hard drive is any faster than if it is connected to USB 3.0. But if there is little difference, than it will likely be more cost effective to stick with mechanical drives for these big backup things where speed is not critical.
And I have to decide about those Cloud backup options like iDisk, Carbonite, etc.