What makes you grow faster in stage 3? Why do girls get their growth spurt in the beginning of puberty, while boys only get it in the middle?
There is a series of hormones that are released at the start of puberty. It results in the production of either testosterone in boys or estrogen in girls. These, combined with other hormones, cause the body to grow.
Both girls and boys grow the fastest in stage 3. Girls typically start puberty about a year before boys. Their stages tend to last only about a year and a half, while boys last about two years. The changes in stage 2 are mostly internal, so there isn't anything very noticeable about a girl's changes until they hit their growth spurt, which on average is about two years before boys. Boys have noticeable changes in stage 2 (spontaneous erections and hair at the base of the penis). The result is that it appears that girls start with a growth spurt, which is not true.