I think I might actually have an answer here, but please correct me if I explain this wrong or go to far off base. LDS believe that Heavenly Father, Jesus Christ, and the Holy Ghost are each divine in nature. Not three seperate Gods, but each having the quality of divinity which is God-like. That is how they kinda come together as one god. They work together as the God Head and are all divine. Just because a being has divinity does not make the being "God," it is simply a God-like quality. Hence one God.
I believe I am using the right term -divinity. But I am actually going to try and use Hinduisms concept of divinity as an example. They have
many idols/ dieties but they are
monotheistic. They believe there is innately divinity in everyone, and that everyone and everything is one with "Brahman"/ God. So.... think of the "substance" of God and existance to be paint. Paint can make skies, earth, animals, people. All different, but all having the same substance and quality. So everything is divine... They have many dieties, but they believe them to all be made of the same substance, along with all of existance, which is the same substance as God. Everyone is not God, but God is in everyone... i.e. divinity.
This is I think.... similar to the idea of the God Head having three divine beings, but yet only one God... which could be considered interchangeably One, between the three, yet three seperate individuals.
{{cringe}}
don't paddle too hard....