One feature of the Mk3 is a single NeoPixel multi-colour LED adjacent to the red power/charge LED.
Unfortunately, during the original manufacture of the badges the NeoPixel was mounted 180 degrees out of orientation. As originally delivered it will not function, being mounted with the triangular notch facing the outer part of the badge and the chip part of the LED (the brown spot under the transparent plastic) facing the charge LED. The correct position is with the triangular notch facing inward and the chip part facing the "D1" label of the silkscreen.
Correcting the faulty mount
The best solution should be to use soldering tweezers take the LED off, clean the pads, turn it, and put it on but this is a delicate operation that will probably destroy the LED. The trick is to be quick, to use a fairly high temperature (350-370 degrees) and add some new solder to the pads to increase the heat transfer.
You can possibly use two soldering irons at the same time, one on each side of the LED.
Avoid hot air at all costs, the badge is packed and you don't really want to risk blowing the charging LED away or even worse, damaging the LCD.
Programming the LED
Or NeoPixelTest in the App Library: http://api.badge.emfcamp.org/app/emuboy/neopixtest