Personalized Mermaid Tail 3rd Birthday Long Sleeve T-Shirt. Cute 3rd birthday Long Sleeve T-Shirt - Charcoal with splashing mermaid tail, seashells and starfish says This Mermaid Is Three. Pre-shrunk 100% cotton 6.1 ounce long sleeve t-shirt. Double-needle stitched neckline and sleeves. Quarter turned. Shoulder-to-shoulder taping. Adult: S, M, L, XL, 2XL, 3XL