Previously Jesus told his hearers that God's throne, footstool, and earthly home are not theirs to swear by. Here he says that they may not swear even by their own heads.
Jesus is talking to a group of people about what they as individuals should and should not do. All occurrences of these words are singular, but you may have to translate them as plural. (See: [[rc://en/ta/man/translate/figs-you]])