1. She still isn’t ready.

2. She still is not ready.
3. She is still not ready.
4. I still don't like chestnuts.
5. I still can't remember them.
6. I still do not like chestnuts.
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This question is about the word order of 'still.'
Are they all correct in terms of word order? Which one is correct, #2 or #3?

They are all correct. Here, "still" is also used correctly. You did not like chestnuts yesterday or last year, and you still don't. :) You didn't remember them and you still do not remember them.