The third Mandala of the Rigveda has 62 hymns, mainly to Agni and Indra. It is one of the "family books", the oldest core of the Rigveda, which were composed in early Vedic period. Most hymns in this book are attributed to viśvāmitra gāthinaḥ. The verse 3.62.10 gained great importance in Hinduism as the Gayatri Mantra.
I have made Indra glorified by these two, heaven and earth, and this prayer of Viśvāmitra protects the people of Bhārata.
Rgveda 3.53.12
Elsewhere, after smiting the Dasyus, he 'gained possession of the sun and horses ... [and] the cow that feeds many (3.34.9).
Destroying the Dasyus, he “gained possession of the sun and horses [...] and the cow of plenty” (3.34.9).
M Danino in History of ancient India / editors, Dilip K. Chakrabarti and Makkhan Lal. v. 3. The texts, political history and administration, till c. 200 BC. I.2. The Horse and the Aryan Debate
Among the Kikatas what do thy cattle? They pour no milky draught, they heat no caldron. Bring thou to us the wealth of Pramaganda;give up to us, O Maghavan, the low-born.
Rigveda 3.53.14, Griffith tr.
Agni, you move to the ocean of Heaven...to the waters which are beyond the luminous heaven of the Sun and to those which stand below it.
Your ancient home, your auspicious friendship, O Heroes, your wealth is on the banks of the Jahnavi.
Rigveda III.58.6. Jahnavi is another name for Ganges in Sanskrit literature and occurs also in Rigveda I.116.19, where it is associated with the Simsumara (I.116.18) [Gangetic river dolphin]. However, Griffith translated it as “the house of Jahnu”. As quoted from Talageri, S. (2000). The Rigveda: A historical analysis. New Delhi: Aditya Prakashan. Chapter 4.
Come forward, Kuśikas, and be attentive; let loose Sudās's horse to win him riches. East, west, and north, let the King slay the foeman, then at earth's choicest place perform his worship.
Rigveda 3.53.11
From the lap of the mountains, happy, smiling, like two running mares, like two bright Mother cows licking their calf, Vipas and Shutudri run with fluid. Directed by Indra, seeking power, as chariots they travel to the sea.
General meaning:
We meditate on that most adored Supreme Lord,
the creator, whose effulgence (divine light) illumines
all realms (physical, mental and spiritual).
May this divine light illumine our intellect.