Personalized Mermaid Tail 3rd Birthday Youth T-Shirt. Cute 3rd birthday Youth T-Shirt - White with splashing mermaid tail, seashells and starfish says This Mermaid Is Three. Let the good times roll in a durable T-Shirt that takes kids from school to play in an amazing array of colors. 5.0 oz., pre-shrunk 100% cotton. Seamless 1x1 rib collar.