Dao de Jing
Ch. 1/81

An interesting note about how its written in Chinese - "dao" is written 3 times in the first line. "Dao" might be synonymous with spirit, but I think you could argue that each dao in the first line is a different version - an eternal dao, a dao with directional properties, and a dao with a more spiritual essense. Kind of like a Trinity...
But the rest clarifies that they are one, and sets the concept within the fractal of perception, where any object can be seen as the three natures. Certainly at least two natures, and maybe three if my speculation is right.
I'm attracted to this model of three in one because its consistent with my own metaphysic of every linguistic object being a symbol of God - which is the three. But again... I could just be seeing what I want to see. In my current formulation, Christianity, Daoism, and Zen Buddhism have a lot of overlap - I'm not even sure if I could find any disagreement between them, beyond modern Christianity's insistence on materialism, which is simply wrong. Bite me, retarded fundamentalists...