1- God is infinite. He cannot be divided. If God has three parts, then he becomes limited and in need, and therefore He can’t be God. God is one.
2- God can send a representative to show us how to pray, but He doesn’t send a part of Himself to do that because he has no parts. And if God makes himself lower to humans, that’s a deficiency in Him. It makes Him limited and in need, and of God becomes in need, He cannot be self sufficient and be God. Yes, God can have mercy on us by sending us great prophets, and He did.