Praise be to Allaah.
If a man has divorced his wife (talaaq) for the first or second time, she spends her iddah (waiting period) in his house and stays with him because she is still his wife and under his guardianship. If he wants to have intercourse with her, then according to some scholars if he does so at that time (i.e. the time of iddah), this means that he has taken her back and her iddah is over. Other scholars say that he should say words that indicate that he has taken her back (e.g., "I take you back" or "I take so-and-so back") before he has intercourse with her, and that there should be two Muslim witnesses to these words. This puts an end to the iddah, and he may then have intercourse with her whenever he wishes. This (the second opinion) is the soundest course to take if he wishes to have intercourse with her. But in the case of the third talaaq, she does not stay with him during her iddah. She leaves his house and is no longer permissible for him (i.e. he cannot have intercourse with her) unless she has married another man; if he divorces her, then she is permitted to return to her first husband in a new marriage. And Allaah knows best.
Sheikh Muhammed Salih Al-Munajjid