Well I think why "three Gods" is excluded is because we are definitively not tritheists. We're Trinitarian, but still monotheists. As for how it is we can believe in three Persons but only one God, well, I'm still looking for a satisfactory answer (I for one do not confess that the living God is merely a lifeless essence, but I'll spare my interpretation as not to derail further). Anyways, it's apparently somewhat of a paradox that not many seem to have an answer for. Since we're talking about truths central to the faith, I thought this belonged, and, well, should serve as a healthy reminder before we go excluding anybody.
I think it's a great question. Particularly since I am finally getting somewhere in understanding it myself - which may or may not make me anyone who should be answering you, LOL.
I'll do my best, but I'm afraid I'm going to fall down when it gets to the deep part of it.
I've always been told "we believe in 1 God in 3 persons - Father, Son, and Holy Spirit". Then they told me the Father had a throne in Heaven, and they told me Jesus became a man, and that He is sitting on another throne at the Father's right hand. So while I could imagine the Holy Spirit as something kind of amorphous, nevertheless, I had two bodily forms and another amorphous form, and they were all "God" and yet I was supposed to believe in only one God.
I nodded my head and agreed, "Yes, 3 persons, 1 God" because that's what I was supposed to do, but deep in my heart, I couldn't understand it. I was told "that's ok, nobody understands" but I still felt guilty that I never admitted, in case I was somehow worshipping three Gods and not One.
Forgive me for my mistakes, and forgive me for that explanation that's going to sound simplistic or possibly ridiculous to some. But that's what I was given, that's where it led, and I AM a fairly logical person with at least a little bit of sense.
Now (taking a deep breath) ... from within the Creed ... I am
SLOWLY getting an inkling of a new understanding. I have a feeling that some of my brothers and sisters in Christ have always understood this. And I have a feeling I'm not going to be able to explain it well. But I'm going to try. Forgive me ... I know my efforts will be less than wonderful.
What is starting to make a great difference to me - is the idea of the Son being begotten of the Father .... and of the Holy Spirit proceeding from the Father.
Wow, I'm stuck already. I know I can say what I don't mean.
I don't mean that these events happened at a point in our time, such that once there was God the Father, then He begat the Son and the Holy Spirit proceeded forth - because that would make the Son and Holy Spirit to be essentially creatures, with a beginning point. There is something I can't quite wrap my mind around with an eternal aspect here, which means that all three are co-eternal (necessary if they are really God).
And the "being of one Substance with the Father" helps me. It is not that God is
divided - that is not correct either.
I have heard the Holy Spirit described as something like breath from the Father, or light as it comes from the Sun - that illustration might help?
Nanopants, I don't know if I am making any sense at all. Please forgive me. I'm not even sure if my difficulties are the same as yours, but I'm just sharing where I was, and trying very hard (without much success) to share what I'm learning, just in case it can help.
I think I should stop. I'm just going to muddle it worse. I'm not sure if this means anything at all to you, or to anyone else who may be reading. And maybe someone else can step in and say this better than I am able to. I've been reading some posts in TAW that have helped, along with others who have explained very patiently to me along the way. It's finally falling into place and making perfect sense to me for the first time (except I'm not saying "I comprehend God" ... I'm saying only "I can understand three Persons, one God). But that understanding is one of the most precious I have gotten among the many wonderful revelations I've gotten in the past year, because finally I can approach God in prayer or worship and know I'm not creating some kind of caricature/idol of my own making.
God bless you. Sorry this is long.