Djago wrote

What exactly are each reading of IO and BR0? What on earth are ATH0, ATH1 and ATH2 on EAP120 and what reading should I expect from them? :confused:

ath N usually designates the physical wireless interface (phy), while wifi N is a virtual wireless interface (one is needed per SSID) on top of the phy. br N is a bridge interface (e.g. eth0 bridged with wifi0). As for IO it could be anything, probably input/ouptut on all interfaces. I also have no idea why the EAP120 lists three phys.

Djago wrote

But I have 3 SSID and only one wifi!

Then it's the other way around despite usual naming conventions.

Is there any way of knowing what is bridging?

https://wiki.debian.org/BridgeNetworkConnections

I'm thinking what you wrote and looking at the numbers... Is there any possibility that on EAP120 each ath N be one SSID?

Yes, could be. Every implementation can assign names in any way. ath usually stands for "Atheros" and this is the driver for the WiFi chip (the physical interface). wl, wlan, wifi are common names for the virtual interfaces on top of the chip's physical interface. Probably it has been reversed for whatever reasons.