"...and I will remember My covenant [with] Jacob, and also My covenant [with] Isaac, and also My covenant [with] Abraham I will remember." [Bechukotai 26:42]
Why are the forefathers listed in reverse order? asked R' Shmelke of Nikolsburg.
Chazal have taught us, answered the Rebbe, that "The world depends on three things - on Torah study, on the service of G-d, and on kind deeds
[Pirkei Avot 1:2]
Each one of the forefathers was noted for a different character trait.
Yaakov embodied Torah study. He was "a wholesome man abiding in tents" [Bereishis 25:27] who studied Torah in the yeshivah of Shem and Ever.
Yitzchak, who had been bound to an altar, represented service of Hashem.
Avraham, the paradigm of hospitality, represented the trait of kindness.
The order in which the verse lists the forefathers - Yaakov, Yitzchak and Avraham - corresponds to the order utilitzed by Chazal to enumerate the three things upon which the world depends: first "Torah", then "service of G-d" and finally "kindness" [Torah, Tefilla, Tzedaka]
Source: Rabbi Yitzchak Bronstein