"Now this time my husband will be joined to me" (Genesis 29:34). The Matriarchs were prophetesses, who knew through their prophecy that twelve tribes were to be established corresponding to the twelve constellations. And she thought that each one of them would have three. Therefore she said, "because I have borne him three sons" — from now on he will have no grievance against me.
Leah Names Levi and the Matriarchs Foresee Twelve Tribes
Midrash Aggadah, Genesis 29:34
הפעם ילוה אישי. האמהות הם הנביאות שידעו בנבואתן שמעמידים י"ב שבטים כנגד י"ב מזלות, והיתה סבורה שיהא לכל אחת מהם שלשה, לכך אמרה כי ילדתי לו שלשה בנים, מעתה לא יהא לו פתחון פה עלי: