$(HUN 6)$ Find the positions of three points $A,B,C$ on the boundary of a unit cube such that $min\{AB,AC,BC\}$ is the greatest possible.
To find the positions of three points \( A, B, C \) on the boundary of a unit cube such that \( \min\{AB, AC, BC\} \) is the greatest possible, we need to consider the distances between these points. 

1. **Identify the possible distances between vertices of a unit cube:**
   - The edge length of the unit cube is \(1\).
   - The face diagonal (distance between two vertices on the same face but not on the same edge) is \( \sqrt{2} \).
   - The space diagonal (distance between two opposite vertices of the cube) is \( \sqrt{3} \).

2. **Consider the minimum distance \( \min\{AB, AC, BC\} \):**
   - For the minimum distance to be maximized, we need to ensure that the smallest distance among \( AB, AC, \) and \( BC \) is as large as possible.

3. **Analyze the possible configurations:**
   - If \( A, B, \) and \( C \) are all vertices of the cube, the minimum distance between any two points can be \(1\), \( \sqrt{2} \), or \( \sqrt{3} \).
   - If \( A, B, \) and \( C \) are on the same face, the minimum distance can be \(1\) or \( \sqrt{2} \).
   - If \( A, B, \) and \( C \) are on different faces but not all vertices, the minimum distance can be \(1\) or \( \sqrt{2} \).

4. **Maximize the minimum distance:**
   - To maximize \( \min\{AB, AC, BC\} \), we need to ensure that the smallest distance among \( AB, AC, \) and \( BC \) is as large as possible.
   - The largest possible minimum distance is \( \sqrt{2} \), which occurs when \( A, B, \) and \( C \) are vertices of the cube such that no two points are on the same edge.

5. **Verify the configuration:**
   - Consider the vertices \( (0,0,0) \), \( (1,1,0) \), and \( (0,1,1) \).
   - The distances are:
     \[
     AB = \sqrt{(1-0)^2 + (1-0)^2 + (0-0)^2} = \sqrt{2}
     \]
     \[
     AC = \sqrt{(0-0)^2 + (1-0)^2 + (1-0)^2} = \sqrt{2}
     \]
     \[
     BC = \sqrt{(1-0)^2 + (1-1)^2 + (0-1)^2} = \sqrt{2}
     \]
   - Thus, \( \min\{AB, AC, BC\} = \sqrt{2} \).

Conclusion:
The positions of the points \( A, B, \) and \( C \) on the boundary of the unit cube that maximize \( \min\{AB, AC, BC\} \) are such that each pair of points is separated by a distance of \( \sqrt{2} \). One such configuration is \( A = (0,0,0) \), \( B = (1,1,0) \), and \( C = (0,1,1) \).

The final answer is \( \boxed{ \sqrt{2} } \).