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Kalidasa
The Sanskrit poet (c. fourth to fifth century CE) whose Kumarasambhava already treats Brahma, Vishnu and Shiva as one form divided in three with interchangeable seniority, the earliest literary witness to the Trimurti idea outside the Puranas.
Sources: Kalidasa on gods — Tamil and Vedas (blog); Kalidasa Part VII: The great integrator — Indica Today; Trimurti — Encyclopaedia Britannica · see Kalidasa's web