I heard that one can become Indra if he does 1000 rajsuvi/ashwamedha yajna(not sure). But is there anywhere in shastras written how can one become one among tridevs?
A Jiva cannot become Ishvara.
Who is Ishvara? Janmadyasyayatah - "From whom is the birth, continuation, and dissolution of the universe," - He is Ishvara - "the Eternal, the Pure, the Ever-Free, the Almighty, the All-Knowing, the All-Merciful, the Teacher of all teachers"; and above all, Sa Ishvarahanirvachaniya-premasvarupah - "He the Lord is, of His own nature, inexpressible Love." These certainly are the definitions of a Personal God. Are there then two Gods - the "Not this, not this," the Sat-chit-ananda, the Existence-knowledge-Bliss of the philosopher, and this God of love of the Bhakta? No it is the same Sat-chit-ananda who is also the God of Love, the impersonal and personal in one. It has always to be understood that the Personal God worshipped by the Bhakta is not separate or different from Brahman. All is Brahman, the One without a second; only the Brahman, as unity or absolute, is too much of an abstraction to be loved and worshipped; so the Bhakta chooses the relative aspect of Brahman, that is Ishvara, the Supreme Ruler. To use a simile: Brahman is as the clay or substance out of which an infinite variety of articles are fashioned. As clay, they are all one; but form or manifestation differentiates them. Before everyone of them was made, they all existed potentially in the clay, and, of course, they are identical substantially; but when formed, and so long as the form remains, they are separate and different; the clay-mouse can never become a clay-elephant, because, as manifestations, form alone makes them what they are, though as unformed clay they are all one. Ishvara is the highest manifestation of the Absolute Reality, or in other words, the highest possible reading of the Absolute by the human mind. Creation is eternal and so also is Ishvara........Those who attain to that state where there is neither knower, nor knowable, nor knowledge, where there is neither I, nor thou, nor he, where there is neither subject, nor object, nor relation, "there, who is seen by whom?" - such persons have gone beyond everything to "where words cannot go nor mind", gone to where the Shrutis declare as "Not this, not this"; but for those who cannot, or will not reach this state, there will inevitably remain the triune vision of the one undifferentiated Brahman as nature, soul and the interpenetrating sustainer of both - Ishvara.
The Complete Works of Swami Vivekananda III.37-42