My advice (although I'm in the Humanities, I think it's pretty standard), is to remember that yo'll be in conversation with two people whom you know have read your thesis with attention to detail. Probably the only two people in the world, apart from you & your supervisor.
So see it as a conversation on a topic in which you are making the claim to be a world expert.
Don't be defensive, as others have said. Know what you thesis is about, AND also what it's not about. And why. Be clear about your choices - and see them and articulate them as choices: methodology, choice of approach, choice of primary evidence, and so on.
If you don't understand a question, ask it back to the Examiners in a calm, uncombative, undefensive way: "Let me check I understand what you're asking me. Do you mean X?"
Take your time. Breathe.
Assume that the three of you are going to have a conversation about a topic you're all invested in.
Have a clear idea where your little bit of research fits into your broader field. And know where you might take that research next.
Good luck!