Ya yēva kartā bhuvanasya hartā samrakshakōvā kathitaḥ purānaḥ, trimūrti rūpēna virājatēyaḥ, tamēka dēvam saranam vrajāmaḥ, tamēka dēvam saranam vrajāmaḥ. Let us understand this shloka. I surrender to the Lord of Lords, the Paramatma, the creator of this entire world, who protects it, who is praised as "Purāna" (the ancient) and the one who has taken the forms of Brahma, Vishnu and Maheshwara.
Sage Atri while doing penance to beget children, meditated to have Paramatma who is in the form of Brahma, Vishnu and Maheshwara, as his offspring. The Trimūrtis appeared at once. Sage Atri asked, "Amongst you three, whom did I call?" Then, Trimūrtis said, "We are the Paramatma upon whom you meditated and hence we will be born to you as your sons." The Moon, Datta and Dūrvāsa were born to Atri as sons. Moon is in the form of Brahma. Datta is in the form of Vishnu and Dūrvāsa is in the form of Shiva. With this incident, we understand that Trimūrtis are one and the same.
In this hymn, the word "Purānaḥ" is used. Whatever meaning "Satyam" conveys in this phrase from Vedas i.e "Satyam Jnānam Anantam Brahma", the same meaning can be described for the word "Purāna" as well. Despite being very ancient, that which appears to be novel at all times is known as "Purānaḥ". God is also known as Purāna. People have been trying since several eons to understand such God and they will continue to do so in the future eons as well. Hence, God is known as "Purāna purusha". Also, poet Kālidāsa described a beautiful hymn to make us understand that there is no difference at all in Trimūrtis. A single form has divided itself into three different forms. It is quite natural for mahātmas to appear superior sometimes and quite inferior at the other. Sometimes, the Puranas say that Shiva is greater than Vishnu; and at some other point of time, they say Vishnu and Brahma are greater than Shiva. At some time, Brahma is considered to be superior to Hari-Hara and vice versa. We must understand these situations and realize that there is no difference at all in Trimūrtis and then we can understand God's form clearly.
Jaya Guru Datta! Sri Guru Datta!