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.