3. As shown in Figure 1, in $\triangle A B C$, $D$ is the midpoint of $A C$, $E$ and $F$ are the trisection points of $B C$, and $A E$ and $A F$ intersect $B D$ at points $M$ and $N$ respectively. Then $B M: M N$ :
$$
N D=(\quad) \text {. }
$$
(A) $3: 2: 1$
(B) $4: 2: 1$
(C) $5: 2: 1$
(D) $5: 3: 2$
3.D.

Connect $D F$, then $B M=M D$. Let $E M=a$, then $D F=2 a$, $A E=4 a$.
Therefore, $A M: D F=3: 2$, which means $M N: N D=3: 2$.
Hence, $B M: M N: N D=5: 3: 2$.