I agree with the response all respondents that no question about that. You should fully address to the feedback of all reviewers whatever their comments, question, suggestion etc. they forwarded regarding you work (PhD thesis). This is because once reviewers are selected and delegated to review your work, they have fully authority to forward what ever comments they believe right. But, you have also the right to disagree to some or all of their comments as far as you have a strong and convincing justifications. However, still I agree with the views of scholars above, you have to first discuss the issues with your supervisors. BY the way, as I am serving MSc and PhD thesis research supervisor and external examiner, I come to know that some students think that they know about their work more than examiners/reviewers and even their supervisors. But eve if there are exceptions, mostly not the case. So, please, I advise you to be careful in this regard.
Following my previous response, an efficient way to address feedback of reviewer, especially in cases of disagreement with them, is to reference other published work/s supporting your point/s.
I agree on Dr. Wassie Haile and others, reviewers they have a full responsibility to handle it; so that must be incorporated the forwarded points since once already the reviewers delegated for reviewing and to give final decision in terms of certification that thesis, they have to accept their comments and decision either the candidates or advises; if not recognized or no value what does matter the defense! The chance of disagreement among them is so rare, but even it's a very small percent its serious issue so as according to my point of view its the assignment of the Institute to establish the Code of Ethics for all of them (candidate, adviser and Reviewers) @ Aristidis Matsoukis.
Depends, if they are genuine comments about the weaknesses, then you must fix them. For comments you are not sure what to do, best person to advise you is the supervisor.