‘And he said to him, “Take on my behalf a three year old heiffer, and a three year old she goat, and a three year old ram, and a turtledove and a young pigeon”.'
As a prophet Abram is commanded to act on God's behalf. As he acts it is Yahweh Who, as it were, takes the animals and birds.
“Three years old”, three, the number of completeness, indicates one which is complete and full. It is the equivalent of ‘without spot and blemish'. What follows takes some considerable period of time, lasting until sundown. Abram has asked, and now he must act deliberately on Yahweh's behalf, not knowing fully what He does.