When a grown-up man has intercourse with a little girl it is nothing, for when the girl is less than this (6) (6. less than three years old) it is as if one puts the finger into the eye (7) (7. tears come to the eye again and again, so does virginity come back to the little girl under three years)
Kethuboth 11b
Pederasty (sex with a boy) with a child below 9 years of age is not deemed as pederasty with a child above that. Samuel said: Pederasty with a child below three years is not treated as with a child above that ... (24) Rab makes nine years the minimum; but if one committed sodomy with a child of lesser age, no guilt is incurred. Samuel makes three the minimum
Sanhedrin 54b
Rabina said: It is therefore to be concluded that a heathen girl from the age of three years and one day, for inasmuch as she is capable of the sexual act she likewise defiles by flux. This is obvious! - You might argue that he is at an age when he knows how to persuade (a female) but she is not at an age when she knows to persuade (a male, and consequently although she is technically capable of the sexual act, she does not cause defilement until she is nine years and one day old)
Abodah Zarah 37a
That a proselyte (convert to Judaism) under the age of three years and one day may be married by a priest.
Yebamoth 60b
A girl of the age of three years and one day may be betrothed by intercourse; if the Yabam (24) (24. The brother of her deceased childless husband) had intercourse with her, he aquires her thereby the guilt of adultory may be incurred through her and she causes uncleanness to the man who had intercourse with her so that he in-turn causes uncleanness to that upon which he lies.
Niddah 44b
A maiden aged three years and a day may be acquired in marriage by coition, and if her deceased husband's brother cohabits with her, she becomes his.
Sanhedrin 55b
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. If you grant that the features disappear and reappear again the blood might well be treated as that of virginity, but if you maintain that they cannot be destroyed until after the age of three years, that must be the blood of menstruation.
Niddah 45a
It is related of Justinia the daughter of 'Aseverus son of Antonius that she once appeared before Rabbi. 'Master', she said to him, 'at what age may a woman marry?'. 'At the age of three years and one day', he told her. 'And at what age is she capable of conception?' 'At the age of twelve years and one day', he replied. 'I', she said to him, 'married at the age of six and bore a child at the age of seven; alas for the three years that I have lost at my father's house'. But can a woman conceive at the age of six years?
Niddah 45a
"When a grown-up man has sexual intercourse with a little girl (49) (49. less than three years old).... their kethubah (money received on the death (or divorce) of her husband) is two hundred zuz (unit of currency)
Kethuboth 11a