The three puruṣa-avatāras are described in Śrīmad-Bhāgavatam. They are also described in the Sātvata-tantra. Viṣṇos tu trīṇi rūpāṇi puruṣākhyāny atho viduḥ: the Supreme Personality of Godhead manifests three features—as Kāraṇodakaśāyī Viṣṇu, Garbhodakaśāyī Viṣṇu and Kṣīrodakaśāyī Viṣṇu—in this material manifestation. The Mahā-Viṣṇu, or Kāraṇodakaśāyī Viṣṇu, is described in the Brahma-saṁhitā (5.47). Yaḥ kāraṇārṇava-jale bhajati sma yoga-nidrām: the Supreme Lord, Kṛṣṇa, the cause of all causes, lies down in the cosmic ocean as Mahā-Viṣṇu. Therefore the Supreme Personality of Godhead is the beginning of this universe, the maintainer of the universal manifestations, and the end of all energy.
(Bhagavad-Gita----10:20----purport).

The Mahā-Viṣṇu, into whom all the innumerable universes enter and from whom they come forth again simply by His breathing process, is a plenary expansion of Kṛṣṇa. Therefore I worship Govinda, Kṛṣṇa, the cause of all causes." Therefore one should conclusively worship the personal form of Kṛṣṇa as the Supreme Personality of Godhead who has eternal bliss and knowledge. He is the source of all forms of Viṣṇu, He is the source of all forms of incarnation, and He is the original Supreme Personality, as confirmed in Bhagavad-gītā.
(Bhagavad-Gita----11:54----purport).

This part of the spiritual sky, called the mahat-tattva, is only an insignificant portion of the whole spiritual sky, and within this mahat-tattva there are innumerable universes. All these universes are collectively produced by the Kāraṇodakaśāyī Viṣṇu, called also the Mahā-Viṣṇu, who simply throws His glance to impregnate the material sky.
(Srimad Bhagavatam----1:3:1----purport).

They can see only the circular sky overhead, without any information that this universe, as well as many other hundreds of thousands of universes, are each covered by sevenfold material coverings of water, fire, air, sky, ego, noumenon and material nature, just like a huge football, pumped and covered, floating on the water of the Causal Ocean, wherein the Lord is lying as Mahā-Viṣṇu. All the universes in seed are emanating from the breathing of the Mahā-Viṣṇu, who is but part of a partial expansion of the Lord, and all the universes presided over by the Brahmās vanish when the Mahā-Viṣṇu withdraws His great breath. In this way, the material worlds are being created and vanished by the supreme will of the Lord.
(Srimad Bhagavatam----2:1:25----purport).

He expands Himself by His different energies, as well as by His plenary portions. In a corner of the spiritual sky of brahmajyoti a spiritual cloud sometimes appears, and the covered portion is called the mahat-tattva. The Lord then, by His plenary portion as Mahā-Viṣṇu, lies down within the water of the mahat-tattva, and the water is called the Causal Ocean (Kāraṇa-jala). While Mahā-Viṣṇu sleeps within the Causal Ocean, innumerable universes are generated along with His breathing. These universes are floating, and they are scattered all over the Causal Ocean. They stay only during the breathing period of Mahā-Viṣṇu. In each and every universal globe, the same Mahā-Viṣṇu enters again as Garbhodakaśāyī Viṣṇu and lies there on the serpentlike Śeṣa incarnation. From His navel sprouts a lotus stem, and on the lotus, Brahmā, the lord of the universe, is born. Brahmā creates all forms of living beings of different shapes in terms of different desires within the universe. He also creates the sun, moon and other demigods.
(Srimad Bhagavatam----2:5:33----purport).

In the Bṛhad-āraṇyaka Upaniṣad (1.4.1) there is the hymn ātmaivedam agra āsīt puruṣa-vidhaḥ. This mantra indicates the Supreme Personality of Godhead (Kṛṣṇa) even before the appearance of the puruṣa incarnation. In the Bhagavad-gītā (15.18) it is said that Lord Kṛṣṇa is Puruṣottama because He is the supreme puruṣa, transcendental even to the puruṣa-akṣara and the puruṣa-kṣara. The akṣara-puruṣa, or the Mahā-Viṣṇu, throws His glance over prakṛti, or material nature, but the Puruṣottama existed even before that. The Bṛhad-āraṇyaka Upaniṣad therefore confirms the statement of the Bhagavad-gītā that Lord Kṛṣṇa is the Supreme Person (Puruṣottama).
(Srimad Bhagavatam----2:9:33----purport).

There are three different types of creation, called mahā-kalpa, vikalpa and kalpa. In the mahā-kalpa the Lord assumes the first puruṣa incarnation as Kāraṇodakaśāyī Viṣṇu with all the potencies of the mahat-tattva and the sixteen principles of creative matter and instruments. The creative instruments are eleven, the ingredients are five, and all of them are products of mahat, or materialistic ego. These creations by the Lord in His feature of Kāraṇodakaśāyī Viṣṇu are called mahā-kalpa. The creation of Brahmā and dispersion of the material ingredients are called vikalpa, and the creation by Brahmā in each day of his life is called kalpa. Therefore each day of Brahmā is called a kalpa, and there are thirty kalpas in terms of Brahmā's days.
(Srimad Bhagavatam----2:10:46----purport).

Every living being is controlled by the supreme living being, Paramātmā, who resides within everyone's heart. He is the puruṣa, the puruṣa-avatāra, who creates this material world. The first puruṣa-avatāra is Mahā-Viṣṇu, and that Mahā-Viṣṇu is the plenary portion of the plenary portion of the Supreme Personality of Godhead, Kṛṣṇa. Kṛṣṇa's first expansion is Baladeva, and His next expansions are Vāsudeva, Saṅkarṣaṇa, Aniruddha and Pradyumna.
(Srimad Bhagavatam----5:11:13-14----purport).

Kṛṣṇa is the origin of all viṣṇu-tattvas, including Mahā-Viṣṇu, Garbhodakaśāyī Viṣṇu and Kṣīrodakaśāyī Viṣṇu. He is the ultimate goal of Vaiṣṇava philosophy. Everything emanates from Him. His body is completely spiritual and is the source of all spiritual being.
(Sri Caitanya Caritamrta----2:20:153----purport).