thirdborn

Adjective

 * 1) Born as the third child to parent or family.
 * Mary, the thirdborn Bennett child in the novel Pride and Prejudice, receives the least attention.
 * 1) Third of a particular type to be born to a parent or family.
 * Prince Arthur was Queen Victoria's thirdborn son and her seventh child overall.

Noun

 * 1) The third child born to a parent or family.