Suppose $ n$ is a positive integer and 3 arbitrary numbers numbers are chosen from the set $ 1,2,3,...,3n+1$ with their sum equal to $ 3n+1$. What is the largest possible product of those 3 numbers?
To solve this problem, we need to find three numbers from the set $\{1, 2, 3, \ldots, 3n+1\}$ such that their sum is $3n+1$ and their product is maximized. Let's denote these three numbers by $a$, $b$, and $c$.

1. **Sum Condition**:
   \[
   a + b + c = 3n + 1
   \]

2. **Product Maximization**:
   We need to maximize the product $abc$ under the constraint $a + b + c = 3n + 1$.

3. **Choosing Numbers**:
   To maximize the product of three numbers, they should be as close to each other as possible. This is because the product of numbers with a given sum is maximized when the numbers are equal or nearly equal.

4. **Finding Closest Numbers**:
   Let's consider the numbers around $n$. We need to find three numbers $a$, $b$, and $c$ such that their sum is $3n + 1$ and they are as close to each other as possible.

   - If we choose $a = n$, $b = n$, and $c = n+1$, then:
     \[
     a + b + c = n + n + (n+1) = 3n + 1
     \]
     The product is:
     \[
     abc = n \cdot n \cdot (n+1) = n^2 \cdot (n+1) = n^3 + n^2
     \]

5. **Verification**:
   We need to verify if there are any other combinations that could give a larger product. Consider other combinations such as $a = n-1$, $b = n$, and $c = n+2$:
   \[
   a + b + c = (n-1) + n + (n+2) = 3n + 1
   \]
   The product is:
   \[
   abc = (n-1) \cdot n \cdot (n+2) = n(n^2 + n - 2) = n^3 + n^2 - 2n
   \]
   Comparing the two products:
   \[
   n^3 + n^2 \quad \text{and} \quad n^3 + n^2 - 2n
   \]
   Clearly, $n^3 + n^2$ is larger than $n^3 + n^2 - 2n$.

Therefore, the largest possible product is obtained when the numbers are $n$, $n$, and $n+1$.

The final answer is $\boxed{n^3 + n^2}$.