Siva (Civa) does not appear therein at all, nor, according to Lassen, is Brahma mentioned in the Vedic hymns, but first in a Upanishad.
Vischnu is spoken of in the Rig-Veda, but always as one of the names for the sun.

He is the Sun-God.

His three steps are sunrise, noon, and sunset.
He is mentioned as one of the sons of Aditi; he is called the "wide-stepping," "measurer of the world," "the strong," "the deliverer," "renewer of life," "who sets in motion the revolutions of time," "a protector," "preserving the highest heaven." Evidently he begins his career in this mythology as the sun.
BRAHMA, at first a word meaning prayer and devotion, becomes in the laws of Manu the primal God, first-born of the creation, from the self-existent being, in the form of a golden egg.

He became the creator of all things by the power of prayer.

In the struggle for ascendency which took place between the priests and the warriors, Brahma naturally became the deity of the former.