What happened in the third year of his surveyorship that changed his future?

This is when the narrator finds the cloth with the embroidered "A" and becomes fascinated with it.

"But the object that most drew my attention in the mysterious package was a certain affair of fine red cloth, much worn and faded. There were traces about it of gold embroidery, which, however, was greatly frayed and defaced; so that none, or very little, of the glitter was left.... It was the capital letter "A."

This discovery makes the narrator return to his passion of writing thus changing his life.