Personalized Superhero 3rd Birthday T-Shirt. Happy 3rd birthday! Celebrate your toddler's third b-day with this cute I'm 3 superhero design. Happy Birthday three-year-old! Everyone loves our Custom T-Shirts! Wardrobe essential for looking good, 24/7. 5.0 oz., pre-shrunk 100% cotton. Seamless 1x1 rib collar. Shoulder-to-shoulder taping. Double-needle stitched sleeves, bottom hem and front neck. Note: greys are a cotton/polyester blend.