#2 is wrong, you can edit your comment after someone replied, but it canbe bad netiquette.
I fix silly typos without warnings (very often).
I make a substancial change, I add "Edit:" or something similar. (Not very often. Most of the times I add another question or source.)
(I also add "Edit:" when I fix a very bad typo noticed in one reply.)
More details in the unofficial feature list maintained by minimaxir https://github.com/minimaxir/hacker-news-undocumented#editde...