Whether a woman becomes a katlanit with the death of two husbands or three husbands: Ketuvot 43b
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The daughter of Rav Chisda chased Chomah (Abbaye's widow) out of town for (accidentally) displaying her beauty, saying, "You've already killed three husbands, now you want to lure another one to kill!": Ketuvot 65a (as explained in Rashi)
