The first instinct of "One" which is also called "Tao", or "God", or whatever-nameless-but-almighty-force is to split into two. Otherwise it is impossible for the One to know that It is. Two as in Yin and Yang, therefore one and the opposite of the same. Three, then it is the manifestation of creative energy to identifiable form.
As Water can be in three forms or density, ice, water, and vapor, so can we speak of ourself in physical, emotional, and spiritual bodies.
We can also speak of passive-plant self, active-animal self, and divine self.
Serenity Prayer, written by Reinhold Niebuhr, used as the opening prayer at Alcoholic Anonymous meeting speaks of the triune so concisely.
"God, grant me the serenity
to accept the things I cannot change, (plant self)
Courage to change the things I can, (animal self)
And wisdom to know the difference. (divine self)