Praise be to Allah.
If a man has divorced his wife 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.
The second opinion is the soundest course to take if he wishes to have intercourse with her. But in the case of the third divorce, 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.
For more, please see this category: Waiting Period of Widow or Divorcee
And Allah knows best.