Please can somebody explain what the various LED lights on the front of the ac3 represent? I've tried plugging and unplugging various things into the ports but can't make heads or tails of it.

Installed 2 of those this week for my customer.
Have to go by memory since they are at least 50km away from where I am now

The lights with the horizontal stripes correspond to eth-ports 1-5.
The blue light in the middle is the WPS button (which I think you can not put out but you can disable the use of that button)
A bit more to the right is (as far as I understood) also network activity (redundant with eth1-5 ?? Not clear)
Far right is wireless connection.

To be honest, I put those 2 devices each time in a cabinet so I was not really concerned with the leds doing whatever they were doing

Those need to be configured. If i remeber correctly, only Blue LED (WPS Button) Ethernet LED (probably switching Activity) and WiFi LED are Always on and hardwired.

The Rest can Display whatever you want, Interface state, activity, script events and so forth.

Go To System -> LEDs and set values accordingly.
i.e. LED1 Interface Status Ethernet 1
LED 2 Interface Status Ethernet 2 etc.

I totally get the confusion with those LED lights. Had the same issue with a new router I got. All these lights flashing and I'm just standing there scratching my head. What helped me was digging up the manual online. It usually explains what each light means. For mine, a solid light was good, but if it blinked, something was up.

If the manual's a no-go, try the old trial and error. Plug something in, see what the light does, unplug it, see if it changes. It's kind of like detective work, but it usually gets the job done.

Also, stumbled upon this site https://leds.to/ while I was trying to figure out a different LED issue. It's got lots of general info about LEDs. Could be handy if you're curious about how these little lights work in other stuff.