And if he has divorced her [for the third time], then she is not lawful to him afterward until [after] she marries a husband other than him.1 And if he [i.e., the latter husband] divorces her [or dies], there is no blame upon them [i.e., the woman and her former husband] for returning to each other if they think that they can keep [within] the limits of Allāh. These are the limits of Allāh, which He makes clear to a people who know [i.e.,understand].
Translation: Saheeh International (English)
1 With the intention of permanence, not merely in order to return to the previous husband.
If the husband divorces her for the third time, he may not remarry her until she gets married to another man in a valid marriage that is genuine and not merely contracted to make her lawful for the original husband, and where sexual intercourse has taken place in this later marriage. Should the second husband divorce her or pass away, the woman and her first husband may return to one another with a new marriage contract and dowry, if they feel that they will fulfil the rights of each other set-out in the sacred law. These rules are made clear by Allah for people who know His laws and limits, as they are the ones who benefit from them.