Is it necessary to accept the statement of the wife's brother, who testifies to the husband divorcing his wife three times, even though the husband does not recall uttering the divorce and the wife did not hear it?

Divorce conditioned on a stipulation with the intent of threatening or emphasizing is a matter of differing rulings. The majority of scholars hold that it takes effect, while some scholars maintain that it does not take effect, but rather requires an expiation for breaking an oath. As for an explicit, immediate divorce, it takes effect whether the husband intended divorce or threat by it. If you pronounced an explicit divorce for the second time, you have completed three divorces, and your wife has become irrevocably divorced from you with a major separation (baynunah kubra). If you are uncertain about whether you pronounced an explicit divorce, then divorce does not take effect with uncertainty. However, if you are predominantly convinced by the truthfulness of those who informed you that you pronounced an explicit divorce, then many scholars have stipulated that one should act upon predominant conviction in matters of divorce. These issues must be presented to trustworthy scholars.