The correct answer to this open question is the following.
Although there are no options attached, we can say the following.
Hinduism's beliefs about many Gods being forms of a single supreme God influence its spread because it was a form to describe the omnipresence of God in many religions that were influenced by this religion.
We are talking about the Hindu religion, the main God can take three aspects. It is the supreme protector of the world and the main preserver of all of creation. It creates, destroys, and purify of the world, at different times, representing a cycle of life. This Hinduist supreme God allows a soul to reborn in some other being. A complicated aspect, difficult to explain in other religions, or non-existent in other belief systems.
That is how the supreme God can take the form of God Brahma, God Shiva, or God Vishnu. A conception spread to other cultures and that became the Trinity in Christian religions as the Father, the Son, and the Holy Spirit.