/docta/ used to be, and remains, primarily /rel/ationships, though now that /dream/ was merged into it, it's a little more haphazard. It's also always been a place to come with mental health issues.
/ask/ has always been more intended to be general questions, with specifics sent to their own board.
/phil/ is obviously for philosophy. I'm not sure where it overlaps heavily with either of the other two.