Do there exist two real monic polynomials $P(x)$ and $Q(x)$ of degree 3,such that the roots of $P(Q(X))$ are nine pairwise distinct nonnegative integers that add up to $72$?
(In a monic polynomial of degree 3, the coefficient of $x^{3}$ is $1$.)
1. **Define the polynomials and their roots:**
   Let \( P(x) \) be a monic polynomial of degree 3 with roots \( z_1, z_2, z_3 \). Therefore, we can write:
   \[
   P(x) = (x - z_1)(x - z_2)(x - z_3)
   \]
   Let \( Q(x) \) be another monic polynomial of degree 3, which can be expressed as:
   \[
   Q(x) = x^3 - sx^2 + tx - u
   \]

2. **Form the composite polynomial \( P(Q(x)) \):**
   The polynomial \( P(Q(x)) \) will have the form:
   \[
   P(Q(x)) = (Q(x) - z_1)(Q(x) - z_2)(Q(x) - z_3)
   \]
   The roots of \( P(Q(x)) \) are the roots of the equations \( Q(x) = z_i \) for \( i = 1, 2, 3 \). Let these roots be \( a_i, b_i, c_i \) for each \( i \), so:
   \[
   Q(a_i) = z_i, \quad Q(b_i) = z_i, \quad Q(c_i) = z_i
   \]

3. **Apply Vieta's formulas:**
   By Vieta's formulas for the polynomial \( Q(x) - z_i \), we have:
   \[
   a_i + b_i + c_i = s
   \]
   \[
   a_i b_i + b_i c_i + c_i a_i = t
   \]
   \[
   a_i b_i c_i = u + z_i
   \]

4. **Sum of the roots:**
   Since the sum of all nine roots of \( P(Q(x)) \) is given to be 72, we have:
   \[
   \sum_{i=1}^3 (a_i + b_i + c_i) = 72
   \]
   Given \( a_i + b_i + c_i = s \) for each \( i \), we get:
   \[
   3s = 72 \implies s = 24
   \]

5. **Determine the roots and coefficients:**
   We need to find three disjoint triples of non-negative integers that add up to 24 and have the same sum of squares. After some experimentation, we find:
   \[
   \begin{aligned}
   &a_1 = 0, \quad b_1 = 11, \quad c_1 = 13 \\
   &a_2 = 1, \quad b_2 = 8, \quad c_2 = 15 \\
   &a_3 = 3, \quad b_3 = 5, \quad c_3 = 16
   \end{aligned}
   \]
   These triples satisfy:
   \[
   a_i + b_i + c_i = 24 \quad \text{and} \quad a_i^2 + b_i^2 + c_i^2 = 576
   \]

6. **Construct the polynomials:**
   Using the values of \( s \) and \( t \), we construct \( Q(x) \):
   \[
   Q(x) = x^3 - 24x^2 + 143x
   \]
   For \( P(x) \), we need roots \( z_1, z_2, z_3 \) such that the product of the roots of \( Q(x) - z_i \) are distinct non-negative integers. We choose:
   \[
   P(x) = x(x - 120)(x - 240)
   \]

7. **Verify the solution:**
   The roots of \( P(Q(x)) \) are distinct non-negative integers that add up to 72, satisfying the problem's conditions.

The final answer is \( \boxed{ P(x) = x(x - 120)(x - 240) } \) and \( Q(x) = x^3 - 24x^2 + 143x \).