Peace be upon you.
Does the third divorce take effect if a husband takes an oath by triple divorce that he will do something and then does not do it out of forgetfulness?
I want to know whether divorce has been effected in such a case? Is there any difference if this happened before or after the conclusion of the marriage contract, knowing that my marriage will take place within a month?
May Allaah reward you.

All perfect praise be to Allaah, the Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

Whoever takes an oath by triple divorce to do something within a certain period of time and then the period lapses without doing it, his wife will be considered divorced thrice according to the opinion of the majority of scholars.

Some scholars are of the opinion that if a husband intends to make divorce contingent upon doing something and then does not do it, his wife will be divorced.

However, if he merely intends to make an oath rather than divorce his wife, he is required to offer the expiation for the broken oath and the bond of marriage will not be affected.

All the previous rulings apply if the oath takes place after the conclusion of the marriage contract.

If a man takes an oath by divorce before the conclusion of the marriage contract, his future wife is not divorced according to the preponderant opinion.

However, such a man should know that swearing by anything other than Allaah The Exalted is prohibited, as the Prophet said: "Whoever takes an oath must swear by Allaah or  remain silent." [Al-Bukhari, Muslim and others]

Allaah Knows best.