וַיְהִ֣י ׀ כְּמִשְׁלֹ֣שׁ חֳדָשִׁ֗ים וַיֻּגַּ֨ד לִֽיהוּדָ֤ה לֵֽאמֹר֙ זָֽנְתָה֙ תָּמָ֣ר כַּלָּתֶ֔ךָ וְגַ֛ם הִנֵּ֥ה הָרָ֖ה לִזְנוּנִ֑ים וַיֹּ֣אמֶר יְהוּדָ֔ה הוֹצִיא֖וּהָ וְתִשָּׂרֵֽף׃
석 달쯤 후에 혹이 유다에게 고하여 가로되 네 며느리 다말이 행음하였고 그 행음함을 인하여 잉태하였느니라 유다가 가로되 그를 끌어 내어 불사르라
Rashi
כמשלש חדשים ABOUT THREE MONTHS AFTER — the greater part of the first, the greater part of the third and the entire middle (second) one (Genesis Rabbah 85:10). The expression כמשלש חדשים signifies “when the months repeated themselves three times”. The word משלש is similar in form to (Ester 9:19) “and sending (ומשלח) portions”; (Isaiah 11:14), “sending (משלח) forth their hand”. Onkelos renders it similarly כתלתות ירחיא (where תלתות is the infinitive of תַּלַּת “when the months become three”).
Chizkuni
זנתה תמר כלתך, “Tamar, your daughterinlaw, has committed an act of infidelity.” She observed the norms of a woman awaiting the levirate marriage, and therefore was out of bounds to any other male pending the resolution of her problematic marital status. She would be released only if her prospective “redeemer” refused to honour this obligation.
Da'at Zekenim
זנתה תמר כלתך, “your daughter-in-law Tamar has committed an act of adultery.” Why did the informant have to add that Tamar had also become pregnant as a result of her act of adultery? Perhaps we can answer that in that era when acts of adultery did not result in the woman becoming pregnant as a result, no legal proceedings were instituted against her. Alternately, proceedings used to be instituted after such a woman had committed adulterous acts with more than one individual.