Hi Aspirant,
N3-,S2-,Cl- are not isoelectronic species as they contain different number of electrons and do not have equal numbers.This is the way to distinguish isoelectronic elements and also it oftens come in the exam.For this you need to have a good knowledge for the atomic number and the periodic table.
Hope it helps!!
Regular exam updates, QnA, Predictors, College Applications & E-books now on your Mobile