Ib #2, 'few' has the idea of 'not many', and , once again we have a negative statement (in its implications), and so an affirmative tag. Note the tense!
In #3, 'a few' has the idea of 'some' - affirmative statement, negative tag.
The apostophe is placed where a letter has been omitted; the correct forms are doesn't, don't, didn't.
It is possible to have an affirmative statement with an affirmative tag, and a negative statement with a negative tag, but we are dealing here with normal usage.,
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