Commentary for Genesis 15:9

עגלה משלשת means THREE HEIFERS: symbolical of three sacrifices of bullocks, viz., the bullock offered on the Day of Atonement, the bullock offered when the correct interpretation of a precept was unknown (העלם) to leaders of the nation (see Leviticus 4:13), and the heifer whose neck had to be broken (see Deuteronomy 21:4) (Genesis Rabbah 44:14).

A HEIFER ‘MESHULESHETH’ AND A SHE-GOAT ‘MESHULESHETH.’ Rabbi Abraham ibn Ezra explained the word meshulesheth as meaning three years old. But Onkelos said “three.”²⁵⁹“Three heifers and three she-goats.” (Onkelos.) This is indeed correct since a three-year old cow is no longer called eglah (heifer), just as we have learned in a Mishnah:²⁶⁰Parah 1:1. And here when the verse says eglah (heifer) it cannot therefore refer to a three-year old, as Ibn Ezra says. “But the Sages say that an eglah (heifer) is two years old; a parah (cow) is three years old.”
The allusion here is to the three sacrifices²⁶¹The heifer, the she-goat and the ram. which his seed will bring from them before Him: the Whole-offering, the Sin-offering and the Peace-offering. And as for the Guilt-offering, that is like the Sin-offering,²⁶²Leviticus 7:7. the difference between them being merely in the name.
It is possible that the meaning of the word meshulesheth is that he bring the three of them consecutively, each kind remaining separate. A similar use of this word is found in the verse, For they were in ‘meshulashoth’ (three) stories,²⁶³Ezekiel 42:6. meaning that there were upper, middle and lower chambers.

קחה לי עגלה, take a heifer for Me, in order to enter into a covenant concerning this assurance. Converting G’d’s promise into what is known as a “covenant,” is equivalent to G’d reinforcing His promise by an oath, something which is unconditional and irrevocable. (Rosh Hashanah 18) Moses himself confirmed this interpretation when he said (Deut 9,5) “not due to your righteousness or the uprightness of your heart, but due to the wickedness of these nations does G’d dispossess them on your account; and in order to keep His promise which He had sworn to your forefathers Avraham, Yitzchok and Yaakov