DS7 went into K with a 2nd grade reading level. His loved magic tree house books at that time. Throughout K, he advanced to about 4th grade level books. This year in 1st, he started out about 6th grade vocabulary and maybe around 4/5 grade reading level. We just took him into a psychologist to do IQ and achievement tests. He is currently around 7th grade vocabulary and 8th grade reading comprehension.
I do not believe in kids reading level even out by 3 grade. I don't even remember if DS was ever in 3rd grade reading level. He seemed to skip over them when he was ready.
For many years, I can see DS's reading and math level jumped and leveled off and jumped again. It is amazing how gifted kids works!