You don't get any time to think nor prepare for part 3 of the speaking test. As soon as the question ends, you should start giving the answer. But since average people need time to process the question, you should start with filler sentences like "This answer to the question that you just asked can be... " or "That is an interesting question and it can be answered with ...." or something similar. If you are not something who can think on his/her feet some time to process the question - which a lot of us do- you can use these genetic sentences that don't require much thinking to give your brain some time to process the question.