Trimurtis are present in every Brahmanda.
This Lord, or Isvara, has three aspects: as Brahma He creates, as Vishnu He preserves, and as Siva He destroys. These three form as one Lord, or God, who is known by three different names according to His three functions. Since the Brahmandas are without number, so too are Brahmas, Vishnus, and Sivas without number. The Purana says: 'One maybe able to count the number of sands on the seashore, but one cannot count the number of the Brahmandas or the deities controlling them.
The Upanishads, Discussion of Brahman in the Upanishads, The Great Lord (Mahesvara), by Swami Nikhilananda
The constituents of creation beginning with mahat (intellect) and ending with Visesa [1] (earth) generate the cosmic egg, whence Brahma arose like a water-bubble.
Linga Purana I.3.28
[1] The group of seven beginning with mahat (intellect) and ending with visesa (bhutas) constitutes the Cosmic Egg which is material, though it derives its potency of consciousness (cetanata) from Purusa.
He alone is Rudra and Vishnu pervading the universe. These worlds are within that cosmic egg and this universe is within it.
Linga Purana I.3.29
The cosmic egg is externally enveloped by water ten times its magnitude. The water is externally enveloped by fire ten times its mass. The air is externally enveloped by the ether ten times its magnitude. The ether is enveloped by ego the cause of sound. The ego is enveloped by intellect and intellect is enveloped by Pradhana. They say that the coverings of the cosmic egg are seven [2]. There within is Brahma seated on the lotus. There are crores and crores of such eggs in existence.
Linga I.3.30-33
[2] The seven sheaths of the Cosmic Egg consist of intellect (mahat), ego (ahankara), and five subtle elements (tanmatras). (Vayu Purana 4.87, Kurma Purana I.4.46)
In all these cosmic eggs there are four-faced Brahmas, Vishnus and Rudras. They are all created by Pradhana after coming into contact with Siva.
Linga Purana I.3.34