I have recently been talking with someone about the Trinity. He asked me to draw the following diagram, where F is the Father, S is the Son and H is the Holy Spirit.
He then asked me to identify Jesus in this diagram. If I picked Jesus as the circle, reflecting my belief that Jesus is God, then I was saying that Jesus is a Trinity. If I picked Jesus as the S, then I was saying that Jesus was not God as God is the circle. The problem is that this diagram misstates the Trinity. I prefer this diagram, where GF is God the Father, GS is God the Son and GH is God the Holy Spirit.
Rather than having one circle with three letters in it, there are actually three circles each with a reference that occupy the same space. This is not tritheism. Tritheism would simply have three circles side by side. Trinity has the three circles together, as Father, Son and Spirit are all fully God and yet are the same God. Nor is this modalism where God takes turns as each person, all three exist and interact at the same time. If asked to identify Jesus in this diagram, I would identify him as the circle with GS in it. He is the circle and he is the GS. Jesus is not a third of God but is fully God.