Yahya related to me from Malik from al-Fudayl ibn Abi Abdullah,the mawla of al-Mahri that al-Qasim ibn Muhammad and Salim ibnAbdullah said, "When a woman is divorced and begins her third period,she is clearly separated from him and is free to marry again."
USC-MSA web (English) reference: Book 29, Hadith 59
Arabic reference: Book 29, Hadith 1220