Read that post already. Door #3 was set to "No Door" by default in the aircraft manager so not sure what door GSX is looking at.
If a door is set to "No Door at this position", is not possible that GSX will ask for it, because that door doesn't even exists, as far as GSX is concerned.
If, instead, you want to use that door, you should set it to "ignore" all open/close checks, so the door will work, but GSX will ignore its status and will not warn you to open/close it.