I did my PhD at Stanford and took all three classes. DB is a more fundamental class, and probably easier. AI and ML are both great but they are designed to be taken in sequence with AI first then ML (which is a sub-field of AI).
Yeah, by 'those topics' I meant the ones covered in the DB course, not the other two. Most aspiring programmers usually find relational theory and XML relatively easy to grok.
[1] writing this I now realize I could easily get last year's course, but I'm satisfied with my current decision