Ok this is going to be a quick and dirty overview of Trinitarian Doctrine. First, we teach that there is One God, not three like the oneness people like to accuse us of teaching. So it boils down to Three Persons in One God, One God in Three Persons. Person does not equal another God. Person does not equal manifestation. Person does equal some sort of seperation from the others with their own set characteristics, most notably the Son, Jesus, is the only one with 100% humanity together with 100% divinity. All three persons were present from eternity because All three are God. How and why there are three persons we do not know because God did not reveal that to us, and He has already admitted to us that no one has seen his fullness because no man could handle the sight.