I mean with a different nusach in the middle beracha, for example on a weekday, we say the the same exact thing 3 times so it would be considered only 1 different shemoneh esrei.
The answer is below
The answer is on Shabbos. Maariv we say אתה קדשת, shacahris we say ישמח משה, mussaf we say תכנת שבת, and mincha we say אתה אחד. The Tur explains why there are different nuschaot for maariv, shacharis, and mincha (mussaf is always different because it needs to focus on the korban mussaf).
The Tur explains that it is כנגד 3 שבתות שבת בראשית, שבת מתן תורה, שבת שבת לעתיד לבא. In fact, this explains the nusach of each of the tefilos.
The maariv שמונה עשרה focuses on creation, we say ויכולו, this is כנגד שבת בראשית. The shacharis שמונה עשרה focuses on how Hashem gave us the mitzva of shabbos at Har Sinai, the שבת מתן תורה (as according to everyone the Torah was given on Shabbos).The mincha שמונה עשרה talks about שבת לעתיד לבא, it says how Hashem will be 1 (see the end of aleinu) meaning that we will understand and see exactly how hashem rules the world. Also we will have peace (מנוחת שלום ושלוה).
I hope this information helps us improve our kavana during davening.
