Hadith.4419 - Hafs ibn Al-Bakhtari narrated from Ishaq ibn Ammar, from Abu Abdullah (as), regarding a man who wishes to marry a woman who has been divorced three times.
Imam (as) said
"He should leave her until she experiences a menstrual period and becomes pure. Then he should go to her husband along with two men and ask him, 'Have you divorced so-and-so?'
If he says yes, he should leave her for three months and then propose marriage to her himself."