You make an interesting point. I think the idea is that N doesn't have to be greater than 3. Although the question has n being at least 3. I think in it's written form, with the ..., N can be one, two, or three. Which is why it is not sufficient.
Also for Data Sufficiency. The two statements cannont be contridictory. So if you were to use like a) and line b) both statements would be contridictory with since the number cannot be prime and even and greater than 3. so your assumption of n>3 is wrong.
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