Ambrose of Milan

St. Luke did not idly present three parables in a row. By the parables of the sheep that strayed and was found, the coin which was lost and was found, and the son who was dead and came to life, we may cure our wounds, being encouraged by a threefold remedy. "A threefold cord will not be broken." Who are the father, the shepherd and the woman? They are God the Father, Christ and the church. Christ carries you on his body, he who took your sins on himself. The church seeks, and the Father receives. The shepherd carries. …

Exposition of the Gospel of Luke ↗

Gregory the Dialogist

For he says: "Which of you, having a hundred sheep, if he loses one of them, does he not leave the ninety-nine in the desert and go after the one that was lost?" Behold, with wonderful dispensation of mercy, the Truth gave a similitude which man might recognize in himself, and yet which pertained especially to the very author of mankind. For since one hundred is a perfect number, he himself had a hundred sheep when he created the substance of angels and men.

Forty Gospel Homilies, Homily 34 ↗

Bonaventure

And he spoke a parable to them. After he manifested the Jewish impiety, so secondly he makes known the divine mercy for confuting the Jewish impiety. And since the mercy of God is especially toward man in the restoration of the human race, in which man needs a reconciling sacrifice, a redeeming price, and an adopting spirit: therefore in this part he introduces a threefold parable; of which the first is about the sheep, which is sacrificed; the second, about the drachma, which is a royal coin; the third is about the father and son, …

Commentary on Luke, Chapter 15 ↗