Praise be to Allah the Lord of the Worlds.
Since scholars differed about this issue, they have reached two opinions:
First: She returns to her first husband and he is entitled to issue her one divorce only. This is the view of the Malikies, Shafies and Hanbalies, in addition to the opinion of Mohammad Bin Hasan Ash-Shaibani and one group of the Hanafie jurists.
Second: She returns to her first husband and he is entitled to issue her three new divorces. This is the opinion of Imam Abu Hanifah, the authors of the Hanafie books and one opinion of Imam Ahmad Bin Hanbal. Kindly check {Fateh Al-Khadeer, vol.4/pp.185} by Ibn Al-Humam, and {Hasheiyat Rud Al-Mohtaar, vol.3/pp.418} by Ibn Abdeen.
In several Muslim countries, Sharia courts follow the opinion of Abi Hanifah, which indicates that a woman`s second marriage annuls all instances of divorce issued to her by first husband. And Allah The Almighty Knows Best.