The correct passive voice sentence is:
(3) A letter is being written by Sunita.

The original sentence is in the active voice and states that "Sunita is writing a letter." In the passive voice, the focus shifts from the doer (Sunita) to the action being done (writing a letter), and the structure typically follows this pattern:

[Object] + [is/are/am] + [past participle of the verb] + [by] + [subject]

So, "a letter" becomes the object, "is being" is used to represent the present continuous tense in passive voice, "written" is the past participle of the verb "write," and "by Sunita" indicates the doer.

Therefore, the passive voice sentence "A letter is being written by Sunita" correctly conveys the same information in a passive construction.