“וַיִּשָּׂא עֵינָיו, וַיַּרְא, וְהִנֵּה שְׁלֹשָׁה אֲנָשִׁים, נִצָּבִים עָלָיו; וַיַּרְא, וַיָּרָץ לִקְרָאתָם מִפֶּתַח הָאֹהֶל, וַיִּשְׁתַּחוּ, אָרְצָה.”
“He lifted up his eyes and looked, and behold, three men were standing in front of him. When he saw them, he ran from the tent door to meet them and bowed himself to the earth”
We studied earlier that the circumcision Abraham underwent gave him the ability to connect Zeir Anpin and Malchut. When that happened, the Zohar explains, Abraham could rise up and see the upper levels. The three men he saw were the aspects of the three columns: Chessed (Abraham), Gevurah (Isaac) and Tiferet (Jacob). They were represented by the angels Michael, Gabriel and Raphael.
These three are the conduits of Light from the upper three Sefirot to the world below. In every silent prayer (the Amida), that is the highest level of spiritual connection, we start by connecting to Abraham, Isaac and Jacob. They are like a spearhead that breaks through all the lower levels so we can connect higher.