Water exists in three states: solid, liquid, and vapor, united as one because they are the same substance: water. God exists in Three Persons: the Father, the Word (Son), and the Holy Spirit, united as one because They are the same Essence: love.
Okay, but be aware that the Creeds use a different definition of "substance" than we do. Today, in modern times, a "substance" is the material from which something is made. When the Creed was developed, a "substance" (Greek: "ousia") is considered the fundamental reality that exists independently and underlies all other properties and changes.
I'll give you an example.
Early in my life, I sat on a wooden chair with four wooden legs and a wooden backrest. Now, I am sitting on a computer chair with four caster wheels mounted to a central, cylindrical leg attached to the bottom of my seat. Both items, easily recognized as a "chair" are different in both design and material composition. The concept of "chair-ness" is universal and does not depend on the particulars or properties of the chair itself. There are hundreds of chair designs and each of them participates in the overall concept of "chair."
Follow me? In Greek thought, the idea of "chair" is the substance of the chair.
Next, consider what would happen to my wooden chair if I removed one of the legs. Although the chair's design has changed, if I redistribute the legs so that they support my weight equally, it will still remain a chair. The "substance" of the chair is its "chair-ness."
Now, I chop up the chair and place it in the fire. The fire will destroy this particular instance of
chair, but the fire will not touch the idea of "chair-ness." And to the Greek mind, the idea of "chair-ness," which is its substance, is indestructible and eternal.
Christians believe that God is indestructible and eternal, but unlike the "ideals" God is a living being who has a will, personality, values, morals and etc. In a sense, God is the most real being that exists. God created "chair-ness" and so God is more real than "chair-ness."
When the Creeds were developed, the debate was whether Jesus was an instantiation of "God-ness", participating in the "ousia" we know as God, or whether Jesus was the hypostatic union of God and Man. Or what? That was the question -- the quiddity of Jesus.
Our question is whether Plato's metaphysics, which forms the basis of the Trinitarian Doctrine is the best explanation of the Biblical witness.