In the given sentence, “He has been living in Varanasi since three years,” the word ‘since’ is incorrect.
The preposition ‘since’ is used to refer to the **starting point of time**, that is, when an action began. It is followed by a specific time or date, such as “since 2020,” “since morning,” or “since Monday.”
However, when we refer to the **duration of time**, that is, how long something has been happening, we use the preposition ‘for’. Since the phrase “three years” refers to a duration, the correct preposition should be ‘for’.
Hence, the corrected sentence is:
“He has been living in Varanasi for three years.”
Therefore, the word that is an error in the sentence is ‘since’.
Final Answer: since