The numbers $1, 2, 3, ... , n$ are written on a blackboard (where $n \ge 3$). A move is to replace two numbers by their sum and non-negative difference. A series of moves makes all the numbers equal $k$. Find all possible $k$
1. **Initial Setup and Goal**: We start with the numbers \(1, 2, 3, \ldots, n\) on the blackboard, where \(n \ge 3\). The goal is to determine all possible values of \(k\) such that after a series of moves, all numbers on the blackboard become equal to \(k\). A move consists of replacing two numbers \(a\) and \(b\) with their sum \(a+b\) and their non-negative difference \(|a-b|\).

2. **Achieving Powers of Two**: We claim that all powers of two greater than or equal to 4 can be achieved. Let's demonstrate this with an example and induction:
   - Start with \((1, 2, 3)\):
     \[
     (1, 2, 3) \rightarrow (2, 2, 4) \rightarrow (0, 4, 4)
     \]
     From \((0, 4, 4)\), we can easily achieve \(k = 4\).

3. **Induction Hypothesis**: Assume that for some \(t \ge 2\), the configuration \((2^t, 2^t, 0)\) is reachable. We need to show that \((2^{t+1}, 2^{t+1}, 0)\) is also reachable.
   - Base Case: For \(t = 2\), we have \((2^2, 2^2, 0) = (4, 4, 0)\), which is reachable as shown above.
   - Inductive Step: Assume \((2^t, 2^t, 0)\) is reachable. Then:
     \[
     (2^t, 2^t, 0) \rightarrow (2^{t+1}, 0, 0) \rightarrow (2^{t+1}, 2^{t+1}, 0)
     \]
     This completes the induction, showing that \((2^t, 2^t, 0)\) is reachable for all \(t \ge 2\).

4. **Maximum Value Argument**: The maximum value of the numbers on the blackboard never decreases. Therefore, any integer \(k \le 2\) cannot be achieved because the initial maximum value is at least 3.

5. **GCD Argument**: The greatest common divisor (gcd) of all numbers on the board either remains constant or doubles after each move. Initially, the gcd of \(1, 2, 3, \ldots, n\) is 1. Since the gcd can only double, it will always be a power of two. Therefore, \(k\) must be a power of two.

6. **Combining Results**: From the above arguments, \(k\) must be a power of two, and it must be at least 4. Therefore, the possible values of \(k\) are all powers of two greater than or equal to 4.

\(\blacksquare\)

The final answer is \( \boxed{ k } \) can be any power of two greater than or equal to 4.