This custom (1) / has come down (2)/ since times immemorial.(3)/ No error.(4)

Answer

3

Explanation

since/from time immemorial will replace since times immemorial because -
since is used for referring back to a previous point in time and it is used as a Preposition with a date, a time or a Noun Phrase.
from/since time immemorial (Idiom): for longer than any one can remember.
Look at the examples given below:
Her family had farmed that land from time immemorial
My home town has had a parade on the fourth of July since time immemorial.