It is a Torah command (Leviticus ch. 23). At the same time, the
sages found many symbolisms in the Lulav and Etrog; one example
being that the Lulav symbolizes the spine, while the Etrog hints to
the heart.
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"bah-RAHK", pronounced just like the president's name.
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Three has many signicances. Here are some examples:
The 3 patriarchs: Abraham, Isaac, Jacob
The world stands on 3 things: Torah, service, deeds of kindness
Every person has 3 names: the one he's born with, the one people call him, and the one he makes for himself.
The 3 functions of a synagogue: house of study, house of prayer, house of assembly