Oh lordy, another "there's a difference, but I can't quite put my finger on it" question. But I'm gonna try anyway. :)
A discussion about a topic — this implies that the discussion was just a conversation, really, and it might not have stayed strictly on-topic.
A discussion of a topic — this brings to mind a true discussion, going into all sorts of details of the topic (and only the topic).
A discussion on a topic — here I picture the discussion to be somewhat one-sided, almost a lecture.
Note that all of these connotations are vague and amorphous, and can be overridden by customary usages, or by what "sounds best" in a given context. If you pressed me to suggest a single best choice, today I'd go with "of". No guarantees about tomorrow.
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