The Hindu Pantheon has a long history, and a confusing one at that due to how old the religion is, as such the mythology around the Hindu Pantheon is in constant debate, here I am just going to put my takes on it, starting at the beginning and adding more as I learn more.
The Hindu Pantheon was started by a deity who right now I just refer to as Brahman/Deva, she is a Void Primordial who does seem to still be active in the pantheon as she has force projected me before (This was unique circumstances, there was a large group of us that had been force projected, I'm not sure how many exactly but I believe every incarnate related to the Hindu Pantheon was). She however has not played a major role in the Hindu Pantheon for a long time from my knowledge, more like just a guiding figure at this point.
Next up there were two deities born, Dyaus, and Prithvi, Dyaus is a cosmic primordial while Prithvi is his counterpart, in the Vedic texts Dyaus is occasionally associated with Rudra, however based on my research through divinations, the etheric, etc, it seems Dyaus became the Trimurti, as in, sharded himself three ways into Shiva, Brahma, and Vishnu. However let's specify when those likely occurred, from my understanding it likely occurred when Indra took over the position of King of the Gods, Dyaus had run the pantheon before him and as his son, Indra would be the next in line, however the circumstances of his rise to the throne are unusual. The Vedic texts seem to say that Dyaus was killed by Indra which this may be the case, there is also a story that may be referring to the same incident, in this story the Vasus (Prithvi, Varuna, Agni, Vayu, Aditya, Dyaus, Soma, and Dhruva) had attempted to steal a divine cow, when caught they were punished to be reborn as mortals and as the children of the deity Ganga, however after some pleading seven of them were freed from the punishment and only Dyaus would serve out the punishment of living as a mortal, my assumption is this marks when Indra took over and the eventual transition to the Trimurti once Dyaus came back to the pantheon after his punishment.
From my divination it appears that Dyaus sharded and became two deities, Rudra, and Vishnu, I'm not entirely sure yet but it seems that Brahma may have a separate origin, however it is worth noting that as of current Brahma is incarnated somewhere and does not have a leadership role within the Hindu Pantheon, in fact the one currently running it is primarily Vishnu, and the Council of Devas.