Augustine of Hippo — as excerpted in the Catena Aurea on Matthew 5:38-42

“(ubi sup.) Let us suppose it therefore said, Go with him other two that the number three might be completed; by which number perfection is signified; that whoever does this might remember that he is fulfilling perfect righteousness. For which reason he conveys this precept under three examples, and in this third example, he adds a twofold measure to the one single measure, that the threefold number may be complete. Or we may so consider as though in enforcing this duty, He had begun with what was easiest to bear, and had advanced gradually. For first He commanded that when the right cheek was smitten we should turn the other also; therein shewing ourselves ready to endure another wrong less than that you have already received. Secondly, to him that would take your coat, he bids you part with your cloak, (or garment, as some copies read,) which is either just as great a loss, or perhaps a little greater. In the third He doubles the additional wrong which He would have us ready to endure. And seeing it is a small thing not to hurt unless you further shew kindnesses, He adds, To him that asketh of thee, give.”