Section 3: A patient arrives with no relevant deficit for the first 72 hours and so does not require any therapy, but then subsequently requires input. How should I record this?

If a patient does not require therapy in the first 72 hours but subsequently requires input you should answer, in Section 3, "no assessment in first 72 hours" and for the reason for not assessing within 72 hours you should selected "patient had no relevant deficit". You can reflect the fact that they did later need therapy in Section 4 of the (post-72h) dataset.