Rashi on Leviticus 19:23/רש"י על ויקרא י״ט:כ״ג

1וערלתם ערלתו את פריו meant lit., ye shall close its closing (regard it as enclosed): the meaning being that it shall be, as it were, closed up and barred so that no benefit may be derived from it.

2שלש שנים יהיה לכם ערלים THREE YEARS IT SHALL BE AS UNCIRCUMCISED UNTO YOU — From when does one count its three years? From the time of its (the tree's) being planted (Sifra, Kedoshim, Section 3 3). One might think that if one has laid it (the fruit) by during these three years it shall become permitted for use after the three years, Scripture therefore states, יהיה which implies it shall remain in its former state of being (forbidden as it was during the entire three years).