Symbolism

DaVinci used symbolism in many of his paintings, this mural is no different.

In the Last Supper you will see DaVinci uses the #3 symbolically through the piece. Jesus is set up in a triangle shape the same as Mona Lisa.

What is special about the #3, what is it referencing in Catholicism?

What are the other 2 uses of 3 in the image?