Brahma and Siva are Fire, in which Vishnu } does not participate, or participates but remotely.
Vishnu and Brahma are Earth, in which Siva
Siva and Vishnu are Water, in which Brahma

In his examination of the Vedas, or Indian Scriptures, Mr. Colebrooke gives the following description of the deities of India:

“The deities invoked appear, upon a cursory inspection of the Veda, to be as various as the authors of the prayers addressed to them: but according to the most ancient annotations on the Indian Scriptures, these various names of persons and things, are all resolvable into different titles of three deities, and ultimately of one God. The Nig’hanti, or Glossary of the Vedas (which is the first part of the Niructa), concludes with three lists of names of deities: the first comprising such as are deemed synonymous with Fire; the second with Air; and the third with the Sun. In the last part of the Niructa, which entirely relates to deities, it is twice asserted that there are but three gods, ’Tisra eva Devatah.’ The further inference, that these intend but one deity, is supported by many passages in the Veda; and is very clearly and concisely stated in the beginning of the index to the Rigveda, on the authority of the Niructa, and of the Veda itself.”

After citing several passages, Mr. Colebrooke continues:—“The deities are only three, whose places are the earth, the intermediate region, and heaven: [namely] Fire, Air, and the Sun. They are pronounced to be [deities] of the mysterious names severally; and (Prajapati) the lord of creatures is [the deity] of them collectively. The syllable O’m intends every deity: it belongs to (Paramasht’hi) him who dwells in the supreme abode; it pertains to (Brahma) the vast one; to (Deva) god; to (Ad’hyatma) the superintending soul. Other deities belonging to those several regions, are portions of the [three] gods; for they are variously named and described on account of their different operations, but [in fact] there is only one deity, the Great Soul (Mahanatma). He is called the Sun; for he is the soul of all beings; [and] that is declared by the Sage. [The Sun] ‘the soul of (jagat) what moves, and of (tast’hush) that which is fixed’; other deities are portions of him: and that is expressly declared by the Sage, ‘The wise call Fire, Indra, Mitra, and Varuna, etc.”

In the Manava Sastra or Institutes of Menu the origin of the Universe is thus unfolded: “It existed only in the first divine idea, yet unexpanded, as if involved in darkness, imperceptible, indefinable, undiscoverable by reason, and undiscovered by revelation, as if it were wholly immersed in sleep. Then the sole self-existing power, who had existed from eternity, shone forth in person, expanding his idea and dispelling the gloom. With a thought he first created the waters, and placed in them a productive seed: this seed became an egg, in which he was himself born in the shape of Brahma, the great forefather of all spirits. The waters are called Nara, because they were the production of Nara, or the spirit of God: and since they were his first Ayana, or place of motion, he was thence named Narayana, or moving in the waters. In that egg the great Power sat inactive a whole year of the Creator: at the close of which, by his thought alone, he caused the egg to divide itself, and from its two divisions framed the world.”

The name given by the Indians to their Supreme Deity, or Monad, is Brahm; and notwithstanding the appearance of materialism in all their sacred books, the Brahmans never admit they uphold such a doctrine, but invest their deities with the highest attributes. He is represented as the Vast One, self-existing, invisible, eternal, imperceptible, the only deity, the great soul, the over-ruling soul, the soul of all beings, and of whom all other deities are but portions. To him no sacrifices were ever offered; but he was adored in silent meditation. He triplicates himself into three persons or powers, Brahma, Vishnu, and Siva, and is designated by the word O’M or rather AUM.

This word O’M is a monosyllable of very profound import. It is supposed to be so holy and awful, like the name Jehovah of the Jews, as not to be guiltlessly pronounced, even by a priest. It must be contemplated, or recited mentally; and it then is said to excite many efficacious aspirations. This awful monosyllable is triliteral, and perhaps therefore better written AUM, for three Sanscrit letters do in fact compose it: but in composition A and U coalesce in O. The first letter is supposed to be symbolical of Brahma, the creative power of the Deity; the second of Vishnu, the preserver; and the last of Siva, the destroyer or renovator. As all the inferior deities of the Hindoos are avataras or manifestation of, and resolve themselves into those three superior powers, so those superior powers resolve themselves ultimately into Brahm, or the supreme being, of whom the sun is the most perfect and glorious murti, or image. A combination of the three symbolical letters forms, therefore, a hieroglyphical representation of the union of the three powers or attributes, and a word that, if uttered, would be nearly expressed by our letters A U M, or O O M, dwelling a little on each letter. A name of Parvati, the consort of Siva, is Uma or Ooma, and it is perhaps hence derivable; as well as Omkar, one of the most sacred places of pilgrimage in India, dedicated to the worship of this mysterious union.

In the Institutes of Menu, many verses occur denoting the importance of this monosyllable, and of a text of the Veda closely connected with it, called the Gayatri. Among those verses are the following:

Chap. ii, v. 74. “A Brahman beginning and ending a lecture on the Veda, must always pronounce to himself the syllable OM: for unless the syllable OM precede, his learning will slip away from him; and unless it follow, nothing will be long retained.” A commentator on this verse says, “As the leaf of the palasa is supported by a single pedicle, so is this universe upheld by the syllable OM, a symbol of the supreme Brahm.”

76. “Brahma milked out as it were, from the three Vedas the letter A, the letter U, and the letter M, which form by their coalition the triliteral monosyllable, together with three mysterious words, bhur, bhuva, and siver.” These words mean earth, sky, and heaven, and are called the vyahritis.