(phrase) how a child acts (at age 3) already determines how he / she will be when he / she is an adult.
Definitions (粵典–words.hk)
語句
人嘅性格好細個就已經定咗，由細細個已經可以睇得出佢將來嘅性格特質 one's character is formed at a young age; "the child is father to the man"; literally: three years old determines eighty years old
Definitions (Wiktionary)
proverb
the character of a person is already formed at a young age