Exercise 3 is Air Experience, there is no legal reason why it should not be conducted at night however, would it normally be conducted at night? The answer to the latter is probably No. You could probably think of a few reasons why not.
A trial lesson is exactly that, a lesson that is conducted as a trial to see if the candidate likes or wants to continue therefore, it is surely in your best interest to conduct it under realistic conditions.
Why does the candidate want to fly at night, is it to see the lights in which case it could be construed to be sightseeing rather than a trial lesson.
Will you brief your student stating that there is a greater risk at night if the engine were to fail, whilst the risk is small and the aircraft doesn't know its dark, there is less chance of a successful landing than if it happend in the daytime.
Finally if you are going to conduct a trial flight at night, it would be Exercise 20 not Exercise 3.