Gittin 3: Can a Woman Witness Her Own Get?

How many people must witness the shaliach's statement that he was present for the signing and giving of the get (2 vs 3)? And why. But isn't one witness enough, under these conditions? Wouldn't a woman also be sufficient under these conditions? Also, whose opinion is the basis for the mishnah's position? Plus, the Torah (d'Oraita) level and rabbinic level to the issues of gittin/divorce.