The myth of the first three years has it that whatever habits, traits, dysfunctions and so on that a child develops during this admittedly formative period are going to stay with him for life. That is not necessarily so. For example, Romanian orphans that suffered severe emotional and physical neglect during infancy and toddlerhood recovered fully after being placed with American families. When put in play groups with American-born kids who were living with biological parents, they could not be identified reliably. The adoption-babblers have a difficult time explaining that, by the way.
Nonetheless, the third birthday is a parenting "hump". Pre-1960s parents understood that the so-called "terrible twos" were just that: to wit, an eighteen-month developmental period (roughly between eighteen and thirty-six months) marked by tantrums, defiance, violent outbursts and other anti-social behaviors. During this same period, it is essential that effective disciplinary precedents be set such that the terrible twos do not become the terrifying threes, frightful fours, fearsome fives, shocking sixes and so on (the nauseating nineteens?).