It is not permissible for a woman who has been thrice-divorced to return to the one who divorced her until she marries another husband who has intercourse with her, then divorces her, and she completes the 'Iddah Sahih Muslim 1433 f
Translation
'A'Asha (Allah be pleased with her) reported
A person divorced his wife by three pronouncements; then another person married her and he also divorced her without having sexual intercourse with her. Then the first husband of her intended to remarry her. It was about such a case that Allah's Messenger (ﷺ) was asked, whereupon he said: No, until the second one has tasted her sweetness as the first one had tasted.