Really, the difference between PTST and the other two (combat PTSD and Complex PTSD) is length of endurance of trauma.
Anyon can get PTSD from a trauma; car crash, medical treatment - anything that frightened them terribly - and caused ptsd symptoms (nightmares, scared of event or place, panic of thoughts about event, avoiance) for 6 months or more. That's the definition.
The other two are ongoing fear, terror, threat of death or presumed death (seeing horrible images, ongoing war, ongoing abuse, etc). The brain becomes like constantly living in a war zone; it expects danger from every corner, nothing feels safe, have to be constantly on guard, shy away from things that normally wouldn't be a problem.... for years.
THen it becomes a neurological thing when the cortosol levels in our brains (for adrenaline) remain high all the time. It affects our health, sleep, appetite... on and on....
short synopsis.