Can I buy/wear new Tzitzis during the three weeks?
Yes. You should wear it for the first time before the beginning of the nine days. Sources: אין מברכים עליו ברכת שהחיינו כמבואר כאן: https://asktherav.com/9770-my-son-seems-eager-and-ready-to-put-on-tzitzis-2-6-years-old-does-he-make-shehechiyanu/ ולכאורה אין קפידא…