John Lightfoot on John 15:3

[Now ye are clean.] Christ having discoursed of the vine and of the branches, these words seem to have an allusion to that law concerning the circumcision of the tree when first planted, Leviticus 19:23. For the first three years the fruit was to be accounted as uncircumcised, unclean, and not to be eaten; "But you, O my branches, now are clean through my word; that word which I have been preaching to you for these three years."