These two verses conclude the opening instructions against ostentatious acts of piety. Verse 4 sums up the point: Those who act on the public stage to receive their rewards of men receive a public reward; those who do good in private are rewarded by God.
Verse 3 emphasizes the need for purity of heart and singleness of purpose in alms-giving. In the ancient world, the left represented evil and the right good. The English word sinister comes from the Latin word, sinistra, but meaning “left.” Certain rabbis (tracing their practice to the venerated Shammai who was roughly contemporary with Jesus) would eat or drink only with the right hand, ostensibly to indicate the purity of their actions. In verse 3, the right hand is acting with such secrecy in alms-giving that the evil left hand does not know what the virtuous right hand is doing. God, the only observer who counts, always knows.
Comparison: There are no changes from the Matthean text. Although Joseph changed the grammatical number (singular to plural) in the previous verse, he did not make the change in this verse. There is no obvious explanation for the difference in translation. The difference does suggest, however, that the change was the result of Joseph’s interaction with the text rather than the Savior’s interaction with his audience. The audience hadn’t changed and there would be no reason for the Savior to make such a change. However, as Joseph translated he would alter them as some aspects as they caught his attention but might easily skip them when they did not.