Adam Clarke on Leviticus 19:23

Three years shall it be as uncircumcised - I see no great reason to seek for mystical meanings in this prohibition. The fruit of a young tree cannot be good; for not having arrived at a state of maturity, the juices cannot be sufficiently elaborated to produce fruit excellent in its kind. The Israelites are commanded not to eat of the fruit of a tree till the fifth year after its planting: in the three first years the fruit is unwholesome; in the fourth year the fruit is holy, it belongs to God, and should be consecrated to him, Lev 19:24; and in the fifth year and afterward the fruit may be employed for common use, Lev 19:25.