Never mind that there are multiple denominations out there. Not going into it, just acknowledging that fact. Some believe each individual is predestined, I for one do not. As for "sin", well God loves his creation so much that he gave man the free will. There were rules, but that damn apple (forbidden fruit and all that) damned us all. Until his son (Jesus) saved us.
I believe in the trinity. And yes, it's God. Who knows what the motives were. Perhaps it's his way of of introspection by taking on three forms?! Who knows.
As for Yahweh, you do know the origins of the his name, right? When Moses asked God what his name was, he responded "I am". Yahweh is "I am" translated in third person. Yahweh (God) is the Alpha and Omega.