You offered --we presume honestly so we are going to answer this honestly: "I don’t understand what you are asking, of course I believe in the Father, The Son and The Holy Spirit. I simply don’t believe they are the same being." All parts of God are the same being and here is how to understand that truth.
If you are a two spatial dimension being (living on a plane of only length and width, no height), could you sense a ball or a pencil? ... Only where that three spatial object intersects you tow spatial variable plane.
Philip said to Jesus, 'Just show us the Father Almighty and that will be enough that we know you are Messiah.'
Jesus answered Philip, 'Have I been so long with you and you and you still do not understand that all you can see of The Father is what you see in me? I am in the father and the Father is in Me. If you have seen me you have seen the Father, for all that you can sense of the father is where He intersects your limits of spacetime. And Philip, that is Me.'
Jesus IS God, intersecting the limits of our sensing. When He appears to take belivers to The Father's House, we will be less limited and we will see Him as He IS. [1John3]
Let's try the classical Christian answer: Given the proviso that you accept that a universal highest power exists according to the Thomist view God is the un-created, first moving necessary being. He lives outside of time and space. So technically the terms before and after having no bearing on the origin of the Son.
Gods first thought was of Himself and that thought was so powerful and perfect that it resulted in the Son. Not as a created being, but rather as a reflection of the Father with all of His divinity. Since the Father already existed the Son's first thought was of Him This first thought was of the perfect love He had for the Father. The Father also immediately had a perfect love for the Son. This love resulted in the third person was the Holy Spirit.
I hope this answers your question.