The third pin is unused. It contributes to firm connection tho.
Many years ago battery sellers started using audio connectors in charge port for safety reasons.
Ping for example. That became a tendence and nowadays many sellers use the 3 pin connectors. I dunno anything about your board but i bet my boots the third pin is unused.
If you plug another brand of charger it will work for sure. Just check +&- and you are golden.
Fans, voltmeters etc have often more than 2 wires. That’s justified. Chargers do not need it. The third pin allows other options though. Temperature for example but again it’s unnecessary for esk8 charge.
I just know the example of the onewheel plug. The third one was there to make sure the board won’t turn on while charging or something like that. Thats why the range Mods people did always left that pin out to trick the board.