[general chem] electron configuration

For the most part I get electron configuration, but this one threw me for a loop! The question was to to write the configuration for I3- doesn't that mean it would gain 3 electrons? How does that work if I usually gains 1 electron? The book said [Kr]5s^2 4p^10 5p^6 or [Xe]? I thought it had to gain 3 and thought it would be [Xe]6s^2

Exact question from book: refer to periodic table and write the predicted electron configuration for each of the following negative ions using core notation: A) F- B) S 2- C) N 3- D) I 3- I understood all except D.