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) $CN=2$