The Torah prohibits one from eating fruits of a tree within its first three years of having been planted. The reason behind this prohibition is explained in Kabbala to be that it is a rectification for the sin of Adam and Eve. G-d had only commanded them to refrain from eating from the tree of knowledge until the beginning of Shabbos, to which only three hours remained until it would commence. Thus our waiting of three years corresponds to the three hours that Adam did not wait and is thus coming to rectify it. In addition, the Chinuch^[1] explains that the fruits of the first three years are unhealthy physically as they are full of liquid. This is similar to other food prohibitions of the Torah which are also unhealthy for a person to eat.

[1]Chinuch 247