A girl who is three years and one day old, whose father arranged her betrothal, is betrothed through intercourse, as the halakhic status of intercourse with her is that of intercourse in all halakhic senses. And in a case where the childless husband of a girl three years and one day old dies, if his brother the yavam engages in intercourse with her, he acquires her as his wife; and if she is married, a man other than her husband is liable for engaging in intercourse with her due to violation of the prohibition against intercourse with a married woman.

Talmud, Niddah 44b