John Calvin Commentary Genesis 15:9

"And he said unto him, Take me a heifer three years old, and a she-goat three years old, and a ram three years old, and a turtle-dove, and a young pigeon." — Genesis 15:9 (ASV)

Take me an heifer of three years old. Some, instead of a heifer of three years old, translate the passage as ‘three heifers’ and, in each species of animals listed, would make the quantity three. Yet the opinion of those who apply the word three to the age of the heifer is more common.

Moreover, although God would not deny his servant what he had asked, he still, by no means, granted what would gratify the desire of the flesh. For, what certainty could be added to the promise by the slaughter of a heifer, or goat, or ram? For the true design of sacrifice, of which we will see more shortly, was until then hidden from Abram.

Therefore, by obeying the command of God, of which, however, no advantage was apparent, he thus demonstrated the obedience of his faith; nor did his wish aim at any other end than this: namely, that the obstacle being removed, he might, as was just, reverently acquiesce in the word of the Lord.

Let us, therefore, meekly learn to embrace those helps which God offers for the confirmation of our faith, although they may not accord with our judgment but rather may seem to be a mockery, until, at last, it becomes plain from the effect that God was as far as possible from mocking us.