This is essentially a matter of stylistic preference and judgment. My preference, and advice, is to use "discussion on [a subject]" as your default. I believe this is the most classic usage. I personally don't like "discussion about", though it is technically correct, because learners often get confused and start using "discuss about", which is wrong. We discuss something. It is wrong to say "We discussed about something." "Discussion as to" should be used, I think, when it is followed by a proposed action, such as "A discussion as to whether we should raise the tax threshold to $50,000 per annum will take place with the Treasury Secretary next week."