Sforno on Genesis 18:2/ספורנו על בראשית י״ח:ב׳

1וישא עיניו, he concentrated on seeing more clearly.

2נצבים עלי, facing him. They appeared as if waiting to speak to him when he would be free to turn his attention from the marvelous manifestation he had just observed. People waiting to speak to someone are described as נצבים עליו, as for instance in Genesis 45,1 להתאפק לכל הנצבים עליו.

3וירץ לקראתם, before they could open a dialogue with him.

4מפתח האהל, he began running from the entrance of his tent. When a person is observed to perform any task with speed, with keenness, this reflects that he considers what he is about to do as important. We have another example of such conduct in Exodus 34,8 וימהר משה ויקוד ארצה וישתחו, “Moses hastened to bow down and to prostrate himself.”

5וישתחו ארצה, seeing that the appearance of these strangers was so awe-inspiring. We have a parallel verse in Judges 13,6 when Manoach’s wife is described as reacting to the angel’s awe-inspiring appearance as such that she did not even dare to enquire his name, etc. Avraham considered these men as emissaries from some king.