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MB3 69b: Davening Ma'ariv and Saying Havdalah Early Shabbos Afternoon (293:3); Havdalah in Shmoneh Esrei (294:1)
Technically, in extenuating circumstances one can davening Maariv and say havdalah (without lighting a candle) after Plag HaMincha on Shabbos afternoon. Ata Chonantanu in Maariv.