The doctrine of the Trinity says that there is one God who eternally exists as three distinct Persons — the Father, Son, and Holy Spirit. Stated differently, God is one in essence and three in person. These definitions express three crucial truths: (1) the Father, Son, and Holy Spirit are distinct Persons, (2) each Person is fully God, (3) there is only one God.
This is beyond Human comprehension. The best example I have is that of visible light. our eyes see only three colors: red, green and blue. yet our brain interprets these as an infinite number of colors and intensities. hence, the color burst:
How many colors do you see in the picture above?
This is an actual picture that I took by shining three lights: red, green, and blue onto a screen.
The Devil: Fallen Angels: