"A maiden aged three years and a day may be acquired in marriage by coition."
Babylonian Talmud Sanhedrin 55b
"That a proselyte (convert to Judaism) under the age of three years and one day may be married by a priest."
Babylonian Talmud Yebamoth 60b
Rather the practical difference is the case, for instance, where her husband had intercourse with her while she was under three years of age and found blood and when he had intercourse after the age of three he also found blood.
Babylonian Talmud Niddah 45a
"For girls, the period of first minority was established, from birth to the age of three, in which a girl was not fit to be married, and the father could not betroth her. From the age of three to twelve was the period of second minority, in which her father could betroth her without her consent."