Dattatreya is an incarnation of Brahma-Vishnu-Maheswara together.
Does Valmiki Ramayana have any explicit mention of Dattatreya?
According to texts, wherein the birth of Sri Dattatreya was mentioned, Anasuya and Atri are his parents.
Anasuya and Atri appear in the fag end of Ayodhya Kanda of Valmiki Ramayana. However, Sri Dattatreya was not even mentioned therein.
So the answer is Sri Dattatreya was not mentioned in Valmiki Ramayana.