to me, I see this, If 1 says evening and morning, that is all we need, We do not need any other day to say that, He could have just said, the second day, the third, We know the first one had an evening and morning, we can easily assume the rest follow the same trend.
The fact that there are three, gives more credibility that we should use them as the example for the rest.
as for added on? not sure what you mean, but to me that would not change anything.
If I tell you a lake has water, I do not have to continue every time I speak of a lake to tell you it has water, the first time, we were told what a lake is, it has water.