The concept of Brahma as Creator, Vishnu as preserver and Siva as destroyer seems to be a Puranic concept. By definition Trimurti cannot appear in Rig Veda since Siva as a God's name does not appear in Rig Veda.
It seems to be a puranic concept
Here they are mentioned in narayana suktam-
tasyāḥ śikhāyā madhye paramātmā vyavasthitaḥ, sa brahma sa śivaḥ sa hariḥ sendraḥ so'kṣaraḥ paramaḥ svarāṭ.
In the middle of that Flame, the Supreme Self dwells. This (Self) is Brahma (the Creator), Siva (the Destroyer), Hari (the Protector), Indra (the Ruler), the Imperishable, the Absolute, the Autonomous Being.
They are not mentioned in rig Veda because one may confuse the rudra and vishnu (vamana) of 33 gods with shiva the supreme rudra and Vishnu from the trinity with the dwarf which many indologists tend to do.