Let $m, n$ be positive integers with $m<n$ and consider an $n\times n$ board from which its upper left $ m\times m$ part has been removed. An example of such board for $n=5$ and $m=2$ is shown below.

Determine for which pairs $(m, n)$ this board can be tiled with $3\times 1$ tiles. Each tile can be positioned either horizontally or vertically so that it covers exactly three squares of the board. The tiles should not overlap and should not cover squares outside of the board.
1. **Determine the total number of squares on the board:**
   The original board is an \( n \times n \) board, which has \( n^2 \) squares. The upper left \( m \times m \) part has been removed, which contains \( m^2 \) squares. Therefore, the number of squares remaining on the board is:
   \[
   n^2 - m^2
   \]

2. **Check the divisibility condition:**
   Each \( 3 \times 1 \) tile covers exactly 3 squares. For the board to be completely tiled with \( 3 \times 1 \) tiles, the total number of squares remaining must be divisible by 3. Therefore, we need:
   \[
   3 \mid (n^2 - m^2)
   \]
   Using the difference of squares, we can rewrite this as:
   \[
   n^2 - m^2 = (n - m)(n + m)
   \]
   Thus, the condition becomes:
   \[
   3 \mid (n - m)(n + m)
   \]

3. **Analyze the divisibility condition:**
   For \( 3 \mid (n - m)(n + m) \), at least one of the factors \( n - m \) or \( n + m \) must be divisible by 3. This gives us two cases to consider:
   - \( 3 \mid (n - m) \)
   - \( 3 \mid (n + m) \)

4. **Check the extension property:**
   If a pair \((n, m)\) satisfies the condition, then adding 3 to both \( n \) and \( m \) will also satisfy the condition. This is because:
   \[
   (n+3)^2 - (m+3)^2 = (n^2 + 6n + 9) - (m^2 + 6m + 9) = n^2 - m^2 + 6(n - m)
   \]
   Since \( 3 \mid (n^2 - m^2) \) and \( 3 \mid 6(n - m) \), it follows that:
   \[
   3 \mid ((n+3)^2 - (m+3)^2)
   \]

5. **Conclusion:**
   The pairs \((m, n)\) for which the board can be tiled with \( 3 \times 1 \) tiles are those for which \( 3 \mid (n - m)(n + m) \). This means that either \( n - m \) or \( n + m \) must be divisible by 3.

The final answer is \( \boxed{ (m, n) } \) such that \( 3 \mid (n - m)(n + m) \).