As explained in the Artscroll: Do not turn to the stubbornness of this people, and to its wickedness and to its sin. Through the merit of the three holy Patriarchs, Moses prays "Do not turn" to three things: by the merit of Abraham, who used the trait of stubbornness to fight idolatry, "do not turn to the stubbornness of this people"; by the merit of Isaac, who willingly bared his neck to the slaughtering knife, "do not turn to its wickedness," the iniquities willfully committed; and by the merit of Jacob, who suffered troubles and exile all his life, "do not turn to its sin," those sins committed unintentionally (for exile atones for unintentional sins). This is why the three items are listed from the most serious fault down to the least serious one, although one would expect the opposite for they are listed in the order of the Patriarchs they correspond to.
|כז זְכֹר, לַעֲבָדֶיךָ--לְאַבְרָהָם לְיִצְחָק, וּלְיַעֲקֹב: אַל-תֵּפֶן, אֶל-קְשִׁי הָעָם הַזֶּה, וְאֶל-רִשְׁעוֹ, וְאֶל-חַטָּאתוֹ.||27 Remember Thy servants, Abraham, Isaac, and Jacob; look not unto the stubbornness of this people, nor to their wickedness, nor to their sin;|
This explanation follows my favorite theme of how one can uplift and channel one's character traits in order to serve God in holiness. And I think it's a really interesting understanding of how precisely each Patriarch helps to counter the people's actions.