There is indeed a subtle difference between the first two.
In a), the reply would be followed by a reason for not answering, eg: 'I'm going to ask John to answer it because I'm sure he knows'. Typical situation - pupil asks question of teacher, who redirects it to another student in order to wake him up!
In b), there is a blunt refusal to answer. It's an odd response, really. More natural would be to say 'I can't answer', meaning that I just don't know.
As for 'I won't be answering it', it's no more polite than anything else. In fact it's rather less polite than a). I'd say it has to be qualified with a time clause, such as 'I won't be answering it today as there's no time left. Ask me again tomorrow.'
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