What you do, meaning making
Istighfar (seeking forgiveness from Allah) three times before the Tashahhud in Salah (Prayer), is a Bid‘ah (innovation in religion) and has no basis in religion. However, it is permissible to make Istighfar three times after Taslim (salutation of peace ending the Prayer) as the Prophet (peace be upon him) did. Besides, it is permissible to make Du‘a’ (supplication) and Istighfar after pronouncing the last Tashahhud and before making Taslim.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.