Rashi on Genesis 15:9/רש"י על בראשית ט״ו:ט׳

1עגלה 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).

2ועז משולשת THREE GOATS — symbolical of the goat the blood of which was sprinkled in the Holy of Holies, of the goats of the additional sacrifices on Festivals, and of the goat brought as a sin-offering by an individual (Genesis Rabbah 44:14).

3ואיל משולש AND THREE RAMS—symbolical of the trespass offering brought by a man who knows for certain that he has committed certain offences, the offering brought by a man who is in doubt whether he has committed such offence, and the ewe brought by an individual as a sin-offering.

4ותור וגוזל means A TURTLE DOVE AND A YOUNG PIGEON (Genesis Rabbah 44:14).