Is there a Hadith or narration that indicates a repentant person must remain steadfast during the first three years because they are the most crucial period?

There is no evidence that the fear of relapse dissipates after three years of repentance. A servant may perform the deeds of the people of Paradise until there is but an arm's length between him and it, then the decree overtakes him, and he performs the deeds of the people of the Fire and enters it. Therefore, he must remain fearful of his sins, anxious lest Allah abandon him to himself, until he meets his Lord. Among the reasons for continuous steadfastness and uprightness is to maintain the practice of catching the first takbeer with the Imam for forty days, for by doing so, one attains immunity from the Fire and hypocrisy.