This is a sentence in poetry or ancient prose, which expresses that a person has been separated from a certain person, place or thing for many years, and his longing for him has become increasingly profound. Specifically, "three years apart" means that we have been apart for three years; "long-awaited" means that we have been missing each other for a long time and deeply; "this day" means that our missing is more profound at this time. The whole sentence means: It has been three years since I was separated from you, and I miss you day by day. This sentence is often used to express people's longing for their hometown, relatives, lovers, friends, etc. after a long absence.