3. As shown in Figure 1, points $A$, $B$, and $C$ are sequentially on line $l$, $M$ is the midpoint of segment $AC$, and $N$ is the midpoint of segment $BC$. To find the length of $MN$, which of the following conditions is sufficient?
Figure 1
(A) $AB=12$
(B) $BC=4$
(C) $AM=5$
(D) $CN=2$
3.A.

Since $M N=M C-N C=\frac{1}{2} A C-\frac{1}{2} B C$
$$
=\frac{1}{2}(A C-B C)=\frac{1}{2} A B,
$$

Therefore, we only need to know $A B$