Good Afternoon IgnatianPhilo: References to the Trimurti are found throughout Hindu texts, but the origins are in a work called the Rig Veda, where God in three forms was established in writing. I say established in writing, because much of Hindu tradition was maintained in the form of careful recitation by holy men until such a time as they were actually written down. Such was also the case with the Upanishads and the Mahābhārata. The Mahābhārata is the epic from which the Bhagavad Gita comes. The Bhagavad Gita is one of the texts of the Mahābhārata, and is largely considered by westerners to be the Hindu equivalent of the Bible. I say widely held by westerners to be such, because Hindus for the most part could care less about creating a nexus between their religion and any other religion, so the task of finding a common reference is left up to westerners. Texts such as the Gita are totally reliant on the idea of the Trimurti as a tenet of their significance. For instance, the Gita is the story of Krishna and his friend Arjuna. Krishna is an Avatar (human incarnation) of the second member of the Trimurti, who is Vishnu. So what you have firmly established in the Bhagavad Gita is the second person of the Hindu Trinity (Vishnu) taking human form for the benefit of humankind, which of course is another similarity between Hinduism and Christianity. And while the authorship of the Gita is widely thought to be roughly contemporaneous with the New Testament (ranging from 300 years prior to 200 years after the New Testament) the cult of Krishna is documented by Greek visitors to the Indian sub continent some 500 years before Christ, and held in oral tradition much earlier than that.
Ideas about trinities or gods in human form are a clearly not a Christian novelty, however, I do not see the tenets of Christianity threatened by any of that. Further, it is impossible to surmise whether the ideas are borrowed or if God simply likes to create different venues on roughly the same themes. The fractal nature of the temporal world suggests that He likes to create diversities from common templates, for instance atoms, planets, solar systems and the like all act in common ways whether at a micro level or macro level, but from basic structures and elements, endless permutations are made possible.