Personalized Superhero 3rd Birthday Youth 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! 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.