3. Given that $n$ is a positive integer, $S=1+2+\cdots+n$. Then the units digit of $S$ cannot be ( ).
(A) $0,1,2,3$
(B) $3,4,5,6$
(C) $3,5,6,8$
(D) $2,4,7,9$
3. D.

Given $S=\frac{n(n+1)}{2}$, and since $n$ and $n+1$ are two consecutive natural numbers, the units digit of $n(n+1)$ can only be $0, 2, 6$. Therefore, the units digit of $S$ can only be $0, 1, 3, 5, 6, 8$. Thus, the units digit of $S$ cannot be $2, 4, 7, 9$.