As per varying scriptures, we get varying accounts.
Even within same scripture we can get varying accounts of Supreme deities.
Since you have given an account from Devi Bhagvat Purana, I'll give another excerpt from the same Purana where Krishna is declared Supreme, only next to the Supreme Parashakti. All the deities are said to ultimately merge into Krishna at the end of Brahma's 100 years, and when all deities hsve been merged into Krishna, he merges into the Supreme Parashakti, as per this excerpt.
Chapter 38, Book 9, DeviBhagvatam
Yamaraja Said -
Then my Father told the Glories of Prakriti Devī. Now I describe to
you what I heard from my Father and what is stated in the Vedas,
though very difficult to comprehend. Listen carefully. O Fair Faced
One! As the eternal space does not know its own extent, so Prakriti
Devī Herself knows not Her own Glories; then what can be said of any
other person on this! She is the Self of all, endowed with all powers
and lordship, the Cause of all causes, the Lord of all, the Origin of
all and the Preserver of all; She is Eternal, always with Her Cosmic
Body, full of everlasting bliss, without any special form,
unrestrained, having no fear, without any disease and decay,
unattached, the Witness of all, the Refuge of all, and Higher than the
Highest; She is with Māyā and She is Mūla Prakriti; the objects
created by Her being known as the Prakriti creation; Who remains as
Prakriti and Puruṣa inseparable from each other as Agni and Her
burning force; the Mahā Māyā, of the nature of everlasting existence,
intelligence and bliss. Though formless, She assumes forms for the
gratification of the desires of Her Bhaktas. She created first the
beautiful form of Gopāla Sundarī, i.e., the form of Śrī Kṛṣṇa very
lovely and beautiful, captivating the mind. His body is blue like the
fresh rain cloud; He is young and dressed like that of a cow herd.
Millions of Kandarpas (the Love deity) are, as it were, playing in his
body. His eyes vie with the midday lotus of the autumn.
....
It is through His fear that the Creator is doing His work of creation
of this Universe; and is recording the Prārabdha fruits of their
Karmas. It is through His fear that Viṣṇu is awarding the fruits of
Tapas and preserving the Universe. By His command the Kālagni Rudra
Deva is destroying all. By Whose favour Śiva has become Mrityumjaya,
the Conqueror of Death and the Foremost of the Jñānins; knowing whom
Śiva has become Himself endowed with knowledge and the Lord of the
knowers of knowledge, full of the Highest Bliss, devotion and
dispassion. Through Whose fear the wind becomes the foremost of
runners and carries things, the Sun gives heat, Indra gives rain, Yama
destroys, Agni burns, and Water cools all the things. By Whose command
the Regents of the (ten) quarters of the sky are watching and
preserving nice orders; through Whose fear the planets are describing
their several orbits. Through Whose fear, trees flower and yield
fruits; By Whose command the Kāla destroys all. By Whose command all
the beings whether on land or in water are quitting their lives in
time; until the proper time comes no man does not die even if he be
pierced whether in battle or in danger. By Whose command the wind
supports the water; the water supports the tortoise; the tortoise
supports the Ananta and the Ananta supports the earth; the earth
supports the oceans, mountain and all the jewels. The earth is of the
nature of forgiveness, i.e., endures all. For this reason all things,
moving and non-moving, rest on Her and again melt away in Her.
Seventy-one Divine Yugas constitute one Indra’s life period.
Twenty-eight Indra’s life periods constitute Brahmā’s one day and one
night. Thus thirty days constitute Brahmā’s one month; so two months
constitute one Ritu (season); six Ritus make one year.
Thus one
hundred years constitute Brahmā’s life. When Brahmā dies, Śrī Hari’s
eye closes. That is the Prākritik Pralaya. At this time, everything,
moving and non-moving, from the Deva loka to Bhūr loka (earth) dies.
The Creator Brahmā gets dissolved in the navel of Śrī Kṛṣṇa. The
four-armed Viṣṇu, of Vaikuṇṭha, sleeps on Kṣīra Samudra, the ocean of
milk, i.e., He dissolves on the left side of Śrī Kṛṣṇa, the Highest
Spirit. All the other Śaktis (forces) dissolve in Mūla Prakriti, the
Māyā of Viṣṇu. The Mūla Prakriti Durgā, the Presiding Deity of Buddhi
(reason) dissolves in the Buddhi of Kṛṣṇa. Skanda, the part of
Nārāyaṇa, dissolves in His breast. Gane’sa, the foremost of the Devas,
born in part of Kṛṣṇa, dissolves in the arm of Śrī Kṛṣṇa.
And those
who are born in parts of Padmā, dissolve in Her body and Padmā
dissolves in the body of Rādhā. All the cow-herdesses and all the
bodies of the Devas dissolve in Rādhā’s body. But Rādhā, the Presiding
Deity of the Prāṇa of Śrī Kṛṣṇa, dissolves in the Prāṇa of Śrī Kṛṣṇa.
Sāvitrī, the four Vedas and all the Śāstras dissolve in Sarasvatī; and
Sarasvatī gets dissolved in the tongue of Śrī Kṛṣṇa, the Highest Self.
The Gopās in the region of Goloka dissolve in the pores of His skin;
the Prāṇa Vāyu of all dissolve in His Prāṇa Vāyu; the fire dissolves
in the fire in His belly; water dissolves in the tip of His tongue,
and the Vaiṣṇavas, (devotees of Viṣṇu), drinking the nectar of Bhakti,
the Essence of all essences, dissolve in His lotus-feet. All smaller
Virāts dissolve in the Great Virāt and the Great Virāt dissolves in
the Body of Śrī Kṛṣṇa. O Child! He is Kṛṣṇa, on the pores of Whose
skin are situated endless Universes; at the closing of Whose eyes, the
Prākritic Pralaya comes and on the opening of Whose eyes, the creation
takes place. The closing and opening of the eyes takes the same time.
Brahmā’s creation lasts one hundred years and the Pralaya lasts one
hundred years. O One of good vows! There is no counting how many
Brahmās or how many creations and dissolutions have taken place. As
one cannot count the number of dusts, so one cannot count the
creations and dissolutions. This is the Great Unspeakable Wonder!
Again on Whose closing of the eyes the Pralaya takes place and on
whose opening of the eyes the creation takes place, out of the will of
God, That Kṛṣṇa dissolves at the time of Pralaya in Prakriti. This
Highest Śakti, the Mūla Prakriti is the Only One without a second; it
is the only one Nirguṇa and the Highest Puruṣa. It is considered as
“Sat” existing, by the Seers of the Vedas. Such a thing as Mūla
Prakriti is the unchanged state (Mukti). During the Pralaya, this only
One Mūla Prakriti appears as Jñāna Śakti or the Knowledge Force
English Translation by Swami Vijnananda
(The Sanskrit + Hindi Translation by the Gita Press Gorakhpur maybe accessed from here)
So here, it has been stated how Krishna manifests from the Supreme Parashakti, who's Nirguna, and from Krishna evolves and in him dissolves every deity.
Now, consider the following excerpt again from the same Purāṇa
Book 5: Chapter 1, DeviBhagvatam
The sages, therefore, consider Viṣṇu superior to Brahmā; they again
consider Rudra superior to Viṣṇu and Māheśvarī (Turīya State) again
superior to Rudra. .... .... Viṣṇu is superior to Brahmā; Rudra is
superior to Viṣṇu (25.a). Therefore no doubt can arise in Kṛṣṇa’s
worshipping Śiva. It is through the will of Śiva that a second Rudra
originated from the forehead of Brahmā to offer boons to him (i. e.,
to Brahmā). This second Rudra is venerable and entitled to all
worship; what to speak of the First Rudra? O King! It is through the
proximity of the Devī that the importance and superiority of Śiva is
thus established (27.b).
So, above, we get yet another different viewpoint.
Now, to answer your question, if the Trinity are immortals or not, that too have varying accounts depending upon the sectarian text being used :
In this question - What is the Life span of Hindu gods
For further reading, refer:
Conclusion -
As per varying texts, depending upon the sect, the interpretation by the acharyas of the sect, etc. we can have all the Trimurtis as either eternal (free from birth-rebirth and karmic cycles), or subject to birth and rebirth, and designated as a position (post).
There's no conclusive pan-Hinduism answer to this question, acceptable by all sects unanimously.
