The basis to this Qs. is that electrons are negatively charged, and protons are positively charged. Essentially, the reason for an atom’s neutrality is a balance between the proton(s)/ positive charge(s) and the electron(s)/negative charge(s). So when 3 electrons are lost, we have an extra 3 protons left. As the charge of the proton is positive, your answer is “3+”.
