There is a way to do a "4th" internet connection. But it isn't as secure.
First -> delete your home internet connection
Second -> Make a homepass router
Third -> Do not setup MAC address changing on a schedule
Now your 3ds will auto connect to the homepass router (attwifi) and you now have a 4th slot. Just one that you can not edit. If you setup Mac address shuffling, every time the Mac address changes you will lose your internet connection. This is a rather annoying problem if you go with some of the larger sets allowing you to change the MAC address every few minutes. So if you use this method, do not have the MAC address change, unless it changes when you will not be online (aka change once a day at midnight)
I have been looking at another possibility, but you could kill the open wifi connections you get at different stores with your 3ds. And until I get my old 3ds back up and running, I'm not willing to test it out. But if you want to take a better look into it it is rather a simple idea.
conf:/hotspot.conf
This file contains a listing of all hotspots the 3ds will connect to. Some do have the security keys. The idea would be to add the extra wifi places you want into this file. In doing so you gain extra "hotspots" that only your 3ds knows how to connect to. Problem is, I don't know if anyone has tried to add any wifi to it. But supposedly the file can contain 100 hotspots, and only around 50 or so are actually used.
If you want more info on hotspot.conf, take a look at:
https://www.3dbrew.org/wiki/Nintendo_Zone