Theophylact of Ohrid

Then were there two thieves crucified with Him, one on the right hand, and the other on the left. As a slander against Christ the two thieves were crucified with Him, so that people would think that He, too, was such a transgressor of the law as they were. The two thieves were symbolic of the two races, the Jewish and the Gentile, for both races had transgressed the law and reviled Christ, just as both these thieves at first reviled Him. …

Commentary on Matthew ↗

Jerome

Then they crucified him with two thieves, one on his right hand, and one on his left. If Golgotha is the hill of Adam and not the place of the condemned, and therefore the Lord is crucified there to raise up Adam, why are the two thieves crucified in that same place?

Commentary on Matthew ↗

Leo the Great

Two thieves were crucified with him, one on the right hand and one on the left, that in the figure of His cross might be represented that separation of all mankind which shall be made in His judgment. The Passion then of Christ contains a sacrament of our salvation, and of that instrument which the wickedness of the Jews provided for His punishment, the power of the Redeemer made a step to glory.

Catena Aurea by Aquinas ↗

Hilary of Poitiers

Or otherwise; Two thieves are set up on His right and left hand, to signify that the entire human race is called to the Sacrament of the Lord's Passion; but because there shall be a division of believers to the right, and unbelievers to the left, one of the two who is set on His right hand is saved by the justification of faith.

Catena Aurea by Aquinas ↗