2) "I have been her kind" means that she closely identifies with each type. This, I believe, is Sexton's transference of her many selves into the poem. She was, in her mind, many different personalities.
3) In the first stanza, the figure is obviously a witch or someone who has beliefs that go against what society dictates, maybe a vixen.
In the second stanza, the figure is the woman who keeps a nicely stocked home is misunderstood for reasons such as the loss of independence.
The last stanza is an allusion (I believe) to Joan of Arc who was burned at the stake for trying to be more than what society deemed a woman should be. Anne Sexton often uses references to her admired her greatly.
Sexton wrote this poem in such a manner that it will continue to speak to and empower women everywhere. She is looking into a looking glass and identifying with all women and with all aspects of her own mentally ill mind. Her state of mind is one of despair, loneliness, and her feelings of being an outcast. Anne Sexton battled depression and dissatisfaction with her place in life all of her life.