Point $M$ lies on the diagonal $BD$ of parallelogram $ABCD$ such that $MD = 3BM$. Lines $AM$ and $BC$ intersect in point $N$. What is the ratio of the area of triangle $MND$ to the area of parallelogram $ABCD$?
1. **Define the coordinates of the parallelogram:**
   Let the parallelogram be a rectangle with vertices \( A = (0, 12) \), \( B = (16, 12) \), \( C = (16, 0) \), and \( D = (0, 0) \).

2. **Determine the coordinates of point \( M \):**
   Since \( M \) lies on the diagonal \( BD \) and \( MD = 3BM \), we need to find the coordinates of \( M \). The diagonal \( BD \) can be parameterized as:
   \[
   M = (x, y) = tB + (1-t)D = t(16, 12) + (1-t)(0, 0) = (16t, 12t)
   \]
   Given \( MD = 3BM \), we have:
   \[
   MD = 3BM \implies 3BM = MD \implies 3 \sqrt{(16t - 0)^2 + (12t - 0)^2} = \sqrt{(16 - 16t)^2 + (12 - 12t)^2}
   \]
   Simplifying, we get:
   \[
   3 \sqrt{256t^2 + 144t^2} = \sqrt{(16 - 16t)^2 + (12 - 12t)^2}
   \]
   \[
   3 \sqrt{400t^2} = \sqrt{(16 - 16t)^2 + (12 - 12t)^2}
   \]
   \[
   3 \cdot 20t = \sqrt{256 - 512t + 256t^2 + 144 - 288t + 144t^2}
   \]
   \[
   60t = \sqrt{400 - 800t + 400t^2}
   \]
   \[
   3600t^2 = 400 - 800t + 400t^2
   \]
   \[
   3200t^2 + 800t - 400 = 0
   \]
   Solving this quadratic equation:
   \[
   t = \frac{-800 \pm \sqrt{800^2 - 4 \cdot 3200 \cdot (-400)}}{2 \cdot 3200}
   \]
   \[
   t = \frac{-800 \pm \sqrt{640000 + 5120000}}{6400}
   \]
   \[
   t = \frac{-800 \pm \sqrt{5760000}}{6400}
   \]
   \[
   t = \frac{-800 \pm 2400}{6400}
   \]
   \[
   t = \frac{1600}{6400} = \frac{1}{4} \quad \text{or} \quad t = \frac{-3200}{6400} = -\frac{1}{2} \quad (\text{not possible})
   \]
   Therefore, \( t = \frac{1}{4} \), and \( M = (16 \cdot \frac{1}{4}, 12 \cdot \frac{1}{4}) = (4, 3) \).

3. **Find the intersection point \( N \) of lines \( AM \) and \( BC \):**
   The equation of line \( AM \) passing through \( A(0, 12) \) and \( M(4, 3) \) is:
   \[
   y - 12 = \frac{3 - 12}{4 - 0}(x - 0) \implies y - 12 = -\frac{9}{4}x \implies y = -\frac{9}{4}x + 12
   \]
   The equation of line \( BC \) is \( x = 16 \).

   Substituting \( x = 16 \) into the equation of \( AM \):
   \[
   y = -\frac{9}{4}(16) + 12 = -36 + 12 = -24
   \]
   Therefore, \( N = (16, -24) \).

4. **Calculate the area of triangle \( MND \) using the Shoelace formula:**
   The vertices of \( \triangle MND \) are \( M(4, 3) \), \( N(16, -24) \), and \( D(0, 0) \).
   \[
   \text{Area} = \frac{1}{2} \left| 4(-24 - 0) + 16(0 - 3) + 0(3 + 24) \right|
   \]
   \[
   = \frac{1}{2} \left| -96 - 48 \right| = \frac{1}{2} \left| -144 \right| = 72
   \]

5. **Calculate the area of parallelogram \( ABCD \):**
   The area of parallelogram \( ABCD \) is:
   \[
   \text{Area} = AB \times BC = 16 \times 12 = 192
   \]

6. **Find the ratio of the area of \( \triangle MND \) to the area of parallelogram \( ABCD \):**
   \[
   \text{Ratio} = \frac{\text{Area of } \triangle MND}{\text{Area of parallelogram } ABCD} = \frac{72}{192} = \frac{3}{8}
   \]

The final answer is \(\boxed{\frac{3}{8}}\).