Basically by asking "How were Trimurthis born?" This question is asking "Who is the originator of whom?" Or "Who is the Ishwara?" Several answers are given here, but most of them are only from their perspective. Here I present all perspectives of all Trimurty.
#Brahma:
Yajurveda Shatapatha Brahmana 6.1.3.1

#Vishnu:

#Shiva:
Svetasvatara Upanishad of Krishna YajurVeda (which is considered among the principle 12 Upanishads) declares this:

#Finally,
Thus from one perspective we see one truth and from another persepective we see another truth.

Mahabharat says this:

Basically by asking "How were Trimurthis born?" This question is asking "Who is the originator of whom?" Or "Who is the Ishwara?" Several answers are given here, but most of them are only from their perspective. Here I present all perspectives of all Trimurty.

Brahma:

Yajurveda Shatapatha Brahmana 6.1.3.1

Vishnu:

Shiva:

Svetasvatara Upanishad of Krishna YajurVeda (which is considered among the principle 12 Upanishads) declares this:

Finally,

Thus from one perspective we see one truth and from another persepective we see another truth.

Mahabharat says this:

Note that above portion of Mahabharata is Jaya section the original 8800 verses of Mahabharata ie. Direct narration by Sanjaya

So from above Lord Shiva is creator of Trimurty.

Note that above portion of Mahabharata is Jaya section the original 8800 verses of Mahabharata ie. Direct narration by Sanjaya

So, from above Lord Shiva is creator of Trimurty.

Remember that Rudra (shiva) has the soul of fire and viShNu has the soul of soma (nectar, moon). All the world including the fixed and movable beings have the combined souls of fire and soma (shiva and viShNu). The supreme lord of rudra (shiva) is viShNu and the supreme lord of viShNu is shiva (rudra). The same lord is moving in the world always in two forms. ViShNu does not exist without ShaNkara (shiva) and Shiva does not exist without Keshava (viShNu). Hence, rudra (shiva) and upendra (ViShNu) have attained oneness, since long before.

Remember that Rudra (Shiva) has the soul of fire and ViShNu has the soul of soma (nectar, moon). All the world including the fixed and movable beings have the combined souls of fire and soma (Shiva and ViShNu). The supreme lord of Rudra (Shiva) is ViShNu and the supreme lord of ViShNu is Shiva (Rudra). The same lord is moving in the world always in two forms. ViShNu does not exist without ShaNkara (Shiva) and Shiva does not exist without Keshava (ViShNu). Hence, Rudra (Shiva) and Upendra (ViShNu) have attained oneness, since long before.