All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon Prophet Mohammad and upon all his family and companions.
Her Iddah* period starts after the third divorce. As for the continued cohabitation, it is unlawful because being ignorant about the rulings of Sharia while in Dar Al-Islam* is no valid excuse. If a baby came as the fruit of this unlawful consummation of marriage, then a judge of Sharia has the final say in this regard because lineage is a serious matter. And Allah The Almighty Knows Best.
* ʿIddah: a specified period of time that must elapse before a Muslim widow or divorcee may legitimately remarry. The Qurʾān (2:228) prescribes that a menstruating woman have three monthly periods before contracting a new marriage; the required delay for a nonmenstruating woman is three lunar months.
* Dar Al-Islam: designates a territory where Muslims are free to practice their religion, though this often implies the implementation of Islamic law, whereas Dar al-Harb represents those lands ruled by non-believers