And he said unto him, Take me (literally, for me, i.e. for my use in sacrifice) an heifer of three years old. So rightly (LXX; Syriac, Samaritan, Arabic, Josephus, Bochart, Rosenmüller, Keil); not three heifers (Onkelos, Jarchi, Kimchi, et alii).
And a she goat of three years old, and a ram of three years old. These offerings, afterwards prescribed by the law (Exodus 29:15; Numbers 15:27; Numbers 19:2; Deuteronomy 21:3), were three in number, and of three years each, to symbolize him who was, and is, and is to come (Wordsworth); perhaps rather to indicate-the perfection of the victim in respect of maturity (Murphy).
Cf. Ganymede's offering (in 'Lucian's Dialogues') of a three years old ram for a ransom. And a turtle-dove, and a young pigeon—also prescribed by the law (Le Genesis 1:14; Luke 2:24).