In Parshas Lech Lecha 12:2, the pasuk famously reads:
וְאֶֽעֶשְׂךָ֙לְג֣וֹיגָּד֔וֹלוַאֲבָ֣רֶכְךָ֔וַאֲגַדְּלָ֖השְׁמֶ֑ךָוֶהְיֵ֖הבְּרָכָֽה׃
I will make of you a great nation, And I will bless you; I will make your name great, And you shall be a blessing.
Rashi there writes:
והיהברכה. הַבְּרָכוֹתנְתוּנוֹתבְּיָדְךָ; עַדעַכְשָׁוהָיוּבְיָדִי, בֵּרַכְתִּילְאָדָםוְנֹחַ, וּמֵעַכְשָׁואַתָּהתְבָרֵךְאֶתאֲשֶׁרתַּחְפֹּץ (בראשיתרבה). דָּבָראַחֵרואעשךלגויגדולזֶהשֶׁאוֹמְרִיםאֱלֹהֵיאַבְרָהָם, ואברכךזֶהשֶׁאוֹמְרִיםאֱלֹהֵייִצְחָק, ואגדלהשמךזֶהשֶׁאוֹמְרִיםאֱלֹהֵייַעֲקֹב. יָכוֹליִהְיוּחוֹתְמִיןבְּכֻלָּן, תַּלְמוּדלוֹמַרוֶהְיֵהבְּרָכָה, בְּךָחוֹתְמִיןוְלֹאבָהֶם
והיהברכה AND BE THOU A BLESSING — Blessings are entrusted to you; hitherto they were in My power — I blessed Adam and Noah — but from now on you shall bless whomsoever you wish (Genesis Rabbah 39:11) Another explanation is: AND I WILL MAKE THEE A GREAT NATION, this alludes to the fact that we say in our prayer “God of Abraham”; AND I WILL BLESS THEE — that we say, “God of Isaac”; AND I WILL MAKE THY NAME GREAT — that we say, “God of Jacob”. One might think that we should conclude the benediction in which these invocations are recited by mentioning again the names of all the patriarchs — the text therefore states “Be thou a blessing” meaning, with you (i.e. with your name only) shall they conclude the benediction and not with them (their names) (Pesachim 117b). (Sefaria translation)
So we see based off the Gemara in Pesachim that rather than conclude the bracha with all the avos, we only do so with Avraham.
The Maharal in Gur Aryeh helps provide a reason why:
בךחותמיןכו'. ואםתאמרמאחרשכלםאבותלמהלאיחתוםבכולם, ואיןזהקשיא, דאיןחותמיןבשתים, כדאמרינןבמסכתברכות (מט.). ואםתאמרמאישנאאברהם, נראהמפנישהבןבכחהאבואיןהאבבכחהבן, ולכךבחתימתאברהםישחתימהיצחקויעקב. ואלודבריםהםאמתלמבינים, והואדברנעלם
With you (i.e. Just Avraham's name) they conclude the Bracha etc. And if you will say since they are all fathers why not sign off with them all (i.e. say Avraham, Yitzchak and Yaakov)? And this is not a problem, since we do not conclude with two (themes). Like we say in Brachos 49a and if you say how is Avraham different it would seem that the son is in the power of the father and the father is not in the power of the son , and therefore included in the signing off of Avraham is both Yitzchak and Yaakov. And these words are true to those who understand them and it is a hidden matter.
So my questions are as follows:
How are the individual Avos to be regarded as a separate themes? Surely, whilst they embodied different core middos e.g. Avraham - chesed, Yitzchak - gevurah and Yaakov - emes/Torah they all still represent the concept of being our forebears, i.e. the Patriarchs who established the standard for us all?
And if we say like the Maharal that the 'son is in the power of the father' and thus Avraham encompasses all of them - why then do we make a point of starting the bracha mentioning all three. Surely there too we could just say Avraham?