Some newer dashcams can connect to an external dashcam-only battery which then feeds off the car's battery only when the car is running. Since such consumer external batteries came to the market much after consumer dashcams were being widely sold, I suspect the 3rd wire is used for handshaking that makes additional features available only to the external battery's management apps, should a user set up his HW kit in that optional configuration. In this case, if a consumer won't be using external batteries at all or if he will be using external batteries but doesn't care to manage, monitor or troubleshoot them via the optional external battery's own app, then the 3rd wire wouldn't be needed. I am no car electronics expert but if anyone in this forum is, then consider this my 2 cents and "food for thought".