According to the talmud, Rosh Hashanah 10b,
- On Rosh Hashanah, Sarah, Rachel and Chanah were designated to give birth, Yosef was released from prison, and the slavery in Egypt ceased.
This is referenced in the Artscroll Rosh Hashanah machzor in the footnotes on page 312 ("She was one of the four childless women...")
But later in the Shacharit (day 1) service, there is a prayer which begins "Em asher b'tzedek nityashna" (page 318). This prayer has the line "v'shalosh akarot shehafk'du b'zeh yom" -- the THREE barren ones who were remembered on this day.
Was it 3 or 4, and why (if it is 4) would this text refer to three?