Answer: The sentence that has a subject-verb agreement error is sentence 3.
Explanation: Rules of subject-verb agreement state that, when "neither...nor" is used in a sentence and both subjects are singular, a singular verb must be used. However, when "neither...nor" is used in a sentence and one subject is singular and the other one is plural, the verb must agree in number with the subject that is nearest to this verb. In that way, sentence 3, which includes "neither...nor", has a subject-verb agreement error because a singular verb was used, even though the subject that is nearest to the verb is plural ("parents"). Therefore, a plural verb ("think") should have been used to give place to subject-verb agreement.