Because 3, is a magic number...
I am very thankful for the Trinity.
Like three parts of an egg, all different yet all together one whole thing.
God created all. Gave people free will, wanting them to crave time with him,a relationship with Him. He gave them the choice, free will. People blew it.
God gave them ways to still have a relationship with Him, if they so chose. Only a few were really, really good at it. Eventually He came down himself, Jesus, his son, who paid the price for all humanity past, present and future and re-connected everyone that so chooses to Father God. Now Jesus couldn't stay here forever, so when he left earth The Comforter, The Holy Spirit,came in His place to help us, guide us.