A child of three? (7)
I believe the answer is:
triplet
'three?' is the definition.
('triplet' can be a synonym of 'three')
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for triplet that I've seen before include "Baby with two similar siblings" , "sibling?" , "One of three born at the same time" , "One of several arriving together" , "One of three at the same birth" .)