פירוש על עבודה זרה 49:18

The Mishnah states that a Jewish woman should not be left alone with a non-Jewish man. The question is—how does this differ from the rule with regard to being left alone with Israelites? The Mishnah in Kiddushin rules that a woman should not be isolated with a Jewish man. Therefore, the prohibition here must refer even to three idolaters. While she can be alone with three Jews, she cannot be alone with three idolaters. [Rashi explains that the Talmud says “three” and not two because a woman is always allowed to be isolated with three Jewish men. Sometimes, she may not be isolated with two. For instance, if they are on the road. If one needed to go to the bathroom, she would be isolated with one].