The research papers I have read indicate that BPD and DID are caused at the same stage in life (childhood & adolesense) and the determining factor of which one is the severity, frequency and duration of trauma experienced by the individual. This is based on the individuals perception as to whether the level reaches a point that splits off a portion of their identity. Also from the papers I have studied, BPD and DID must be early onset to be considered, as in must begin in childhood or early adolesence. Where as PTSD could be caused and experienced at any age.

I would think PTSD would co-occur with either of BPD, DD or DID as they are both rooted in terrible experiences and if some is bad enough to cause a dossociative split, then I would think there would be PTSD envolved.

Something to keep in mind with DID is that many psychologists/psychiatrists do not acknowledge the existence of it, and think it is an act. Though it is in the DSM, and I personally believe whole heartedly that it is real. I do think it is possible to have it an d not know. Though there would be signs like missing chunks of time, things moved around or bought that you dont remember, etc. It would be necessary for your professional diagnosing you to believe DID is a real disorder to really get an honest opinion on if you have it or not.

I would think that a person with both PTSD and DD or DID may even dissociate or switch to another alter when something triggered a PTSD flashback.

This is an actual research study done on the exact questions you pose. It is a technical read, but I found it to be awesome in covering the relationship of the disorders and ptsd.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4579511/