Yes, horses will grow for their entire lives. Typically horses are considered fully grown between 5 and 7-years old. However, horses can continue growing until they reach 10 years of age. The growth rate slows down significantly after 3 years of age, but horses will still put on some weight and muscle as they get older. Proper nutrition and care are important for optimal growth at any age.
So, if you have a 3-year-old horse, don't worry - they will continue growing until they reach their full potential. Make sure to provide them with a good diet and plenty of exercise so they can reach their maximum growth rate.