Re: capitalization of pronouns...I don't care.
As for your last sentence...Christians didn't "invent" a Trinity in order to pull off their claim to be monotheistic, they developed the defining of it to show that this was revealed in God's word repeatedly. Scripture says the Lord God (Jehovah/Hashem/etc.) is the ONLY true God and besides him, there is no other God/Savior. The Father is God/Savior/Creator. Jesus/Son of God, Scripture says, is the Savior/Redeemer, the creator of everything, God with us. It also says the Holy Spirit is God. So, since we believe there are not three gods, but only ONE God, the Father, the Son and the Holy Spirit must all be the parts of the "Godhead". We really are impotent with fallible minds trying to comprehend the infinite God. That is why it must be taken by faith - because it is unmistakable that God HAS revealed this truth about himself regardless whether we understand it or not. Some people accuse Christians of copping out by saying it's a mystery we accept by faith, but it really is NOT a cop out. It's the truth that we take on faith just like a great many other things about God we can't fully understand.
By man's reasoning perhaps, since as you say, "We really are impotent with fallible minds trying to comprehend the infinite God.", which I'm in total agreement with you on, BTW.
However many parts one believes makes up the Spirit of God, it is that Spirit we follow in allowing others room to believe differently.