Parashas Vayeira · Passuk 18:2
He lifted his eyes and saw, and behold, three men were standing over him; he saw, and he ran toward them from the entrance of the tent, and he bowed to the ground.
Torah Temimah points to Yoma 37a.
TT lemma: נצבים עליו.
English: “Standing over him.” When three walk along the way, the teacher is in the middle, the greater [disciple] on his right and the lesser on his left. So we find with the three angels who came to Avraham: Michael in the middle, Gabriel on his right, and Raphael on his left.
Derash on the verse (Bavli sugya) — not a formal named middah of the 13. The phrase “standing over him” is taken to imply that they stood in a single line, with the greatest in the middle.
When three walk together, the most important person walks in the middle; the three angels who visited Avraham stood in that order, Michael central.