Can someone have schizophrenia, bipolar and multiple personality disorder at the same time?

Well, First off -: Multiple personality disorder is rare. Second - if there is a mix of bipolar ; psychotic sxs - it might be schizoaffective disorder. I think it would be unlikely to have this particular combination together. There are many similiary symptoms that can occur with 3 disorders. It could be a confusing picture.

Depends on how: many doctors are involved at the same time. Because symptoms for these 3 things overlap, it is unhelpful for a single doctor to diagnose all three of these for the same person all at the same time.