Here Joseph Smith, in his editing for the 1837 edition, initially interpreted the relative pronoun which as referring to the immediately preceding “our first parents”, and thus he edited which was to who were. But then Joseph realized that which referred either to the earlier “that old serpent” or, more accurately, to the beguiling itself; thus he restored the original which was. Interestingly, there is one other passage in the original text where the antecedent for the relative pronoun which is clearly “that old serpent”, and in that passage the which was edited to who in the 1837 edition:
The critical text will, of course, maintain the original which in both 2 Nephi 2:18 and Mosiah 16:3. For discussion regarding which when the antecedent is “the devil” or “the evil spirit”, see under Mosiah 4:14. For general discussion, see under which in volume 3.
Summary: Maintain the use of the relative pronoun which in Mosiah 16:3; the which refers to “that old serpent” or to the fact that Satan beguiled Adam and Eve.