A triangle $ABC$ $(a>b>c)$ is given. Its incenter $I$ and the touching points $K, N$ of the incircle with $BC$ and $AC$ respectively are marked. Construct a segment with length $a-c$ using only a ruler and drawing at most three lines.
1. **Identify the given elements:**
   - Triangle \(ABC\) with sides \(a = BC\), \(b = CA\), and \(c = AB\) such that \(a > b > c\).
   - Incenter \(I\) of the triangle.
   - Points \(K\) and \(N\) where the incircle touches sides \(BC\) and \(AC\) respectively.

2. **Draw the line segment \(AN\):**
   - Since \(N\) is the point where the incircle touches \(AC\), draw the line segment \(AN\).

3. **Draw the line segment \(IK\):**
   - Since \(K\) is the point where the incircle touches \(BC\), draw the line segment \(IK\).

4. **Construct the line segment \(AK\):**
   - Draw the line segment \(AK\). Note that \(AK\) is tangent to the incircle at \(K\).

5. **Use the Iran Lemma:**
   - According to the Iran Lemma, in a triangle \(ABC\) with incenter \(I\) and incircle touching \(BC\) at \(K\) and \(AC\) at \(N\), the length of the segment \(AK\) is equal to \(s - b\), where \(s\) is the semiperimeter of the triangle.
   - The semiperimeter \(s\) is given by:
     \[
     s = \frac{a + b + c}{2}
     \]
   - Therefore, the length of \(AK\) is:
     \[
     AK = s - b = \frac{a + b + c}{2} - b = \frac{a - b + c}{2}
     \]

6. **Construct the line segment \(AN\):**
   - Similarly, the length of the segment \(AN\) is equal to \(s - c\):
     \[
     AN = s - c = \frac{a + b + c}{2} - c = \frac{a + b - c}{2}
     \]

7. **Construct the segment \(a - c\):**
   - To find the segment of length \(a - c\), we can use the difference between \(AK\) and \(AN\):
     \[
     AK - AN = \left(\frac{a - b + c}{2}\right) - \left(\frac{a + b - c}{2}\right) = \frac{a - b + c - a - b + c}{2} = \frac{2c - 2b}{2} = c - b
     \]
   - However, we need \(a - c\), not \(c - b\). To achieve this, we can use the fact that:
     \[
     a - c = (a - b) + (b - c)
     \]
   - We already have \(AK = \frac{a - b + c}{2}\) and \(AN = \frac{a + b - c}{2}\). By drawing the line segment \(AK\) and extending it to the point where it intersects the line through \(N\) parallel to \(BC\), we can construct the segment \(a - c\).

The final answer is \(\boxed{a - c}\).