Resh Lakish said: "And I will make you a great nation" (Genesis 12:2), this is what we say in prayer, "the God of Abraham." "And I will bless you," this is what we say, "the God of Isaac." "And I will make your name great," this is what we say, "the God of Jacob." One might think that they should seal the blessing with all of them; therefore Scripture teaches, "and be a blessing" (Genesis 12:2): with you alone they seal, and they do not seal with all of them. [That is, the first benediction of the Amidah opens by naming all three patriarchs but closes only with "Shield of Abraham."]
Three Blessings That Became the Names of the Patriarchs in Prayer
Yalkut Shimoni on Torah 64:3
אָמַר רֵישׁ לָקִישׁ: וְאֶעֶשְׂךָ לְגוֹי גָּדוֹל, זֶה שֶׁאוֹמְרִים 'אֱלֹהֵי אַבְרָהָם'. וַאֲבָרֶכְכָה, זֶה שֶׁאוֹמְרִים 'אֱלֹהֵי יִצְחָק'. וַאֲגַדְּלָה שְׁמֶךָ, זֶה שֶׁאוֹמְרִים 'אֱלֹהֵי יַעֲקֹב'. יָכוֹל יְהוּ חוֹתְמִין בְּכֻלָּן, תַּלְמוּד לוֹמַר: וֶהְיֵה בְּרָכָה, בְּךָ חוֹתְמִין, וְאֵין חוֹתְמִין בְּכֻלָּן.