Genèse 15:9

And He said, Bring Me oblations, and offer before Me an heifer of three years, and a goat of three years, a ram of three years, and a dove, and the young of a pigeon.

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.” This is indeed correct since a three-year old cow is no longer called eglah (heifer), just as we have learned in a Mishnah: (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 sacrifices 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, 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, 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

'ויאמר אליו קחה לי וגו, the purpose of Avram taking these animals was to offer them to Gd as a sacrifice. G’d would use this sacrifice to illustrate to Avram the future of his descendants, their settlement in the land of Canaan, as well as their exile from that land. By means of these animal sacrifices G’d would conclude a covenant with Avram that was not subject to permanent dissolution, ever. Concluding a covenant is a procedure such as follows in the next verse, ויבתר אותם בתוך. In Jeremiah 34,18 the prophet spells this out more clearly, עגלה אשר כרתו לשנים ויעבר בין בתריו, “(like) the calf which they cut in two so as to pass between the halves”

the Torah mentions three species of animals (mammals) which are basically fit to be offered as sacrificial animals, hinting thereby that as long as these kinds of animals would be offered on the altar to G’d in the prescribed manner, the Jewish people, Avram’s descendants, would not be exiled from their land. The reason why 3 of each species were to be offered by Avram at this time was to apprise him of the fact there would indeed occur three exiles when his descendants would be forcefully uprooted from their land. Their first such exile would be to the land of Egypt, also known as עגלה, calf (Jeremiah 46,20) עגלה יפה פיה מצרים, “Egypt is a handsome heifer.” In Psalms 80,6 the psalmist writes: ותשקמו בדמעות שליש, “You have fed them tears three times,” a prophetic prediction by the author that the exile of the Jewish people in Egypt would not be their only one, but that two more were to follow.

עגלה, this word was an allusion to the first exile of the Israelites. Seeing that Yaakov’s sons had already occupied prominent positions in the land of Canaan, the land could have been described as theirs, if not as their inheritance, as G’d said in Genesis 15,16 “for the measure of the Emorites’ sin will not be full until 4 generations from now.” G’d would not have a legal justification for expelling them from their land until after that period of time. [Most Midrashim understand עגלה as a reference to the Babylonian exile, compare Torah Shleymah on our verse. Ed.]